This will cover mine and others interpretation of the lore for the bosses in the Dark Souls II DLC. Please share your interpretations and any information I've missed below.

My other lore guide is here: https://steamproxy.net/sharedfiles/filedetails/?id=1436671498
Any help with typos or errors is much appreciated! :D

Aava appears to be a saber-tooth tiger. It was one of the seven beasts who served the Ivory King. Each of these beasts was given a specific task. We know of only 3 beasts that remain; Aava, Lud and Zallen. Aava’s task was the guardianship of the King’s beloved child of Dark; Alsanna. Lud and Zallen were tasked with the mercy killing of exiles. From this we know that the beasts were given a broad range of tasks, from protecting life to taking it.

Since the Ivory King was served by 7 of these massive beasts, it indicates he must have been very powerful. Aava is invisible to the naked eye. The only way to actually see Aava is to look through the Eye of the Priestess, an eye of the first priestess who watched over Eleum Loyce. These priestesses worked to contain the Old Chaos but by the time the Player reaches Eleum Loyce, the line of priestesses has been extinguished.

Aava’s soul can be crafted into the Ivory Straight Sword. What connection Aava may have to Sir Fabian is unknown, as Sir Fabian was said to have delved into Chaos in an attempt to exterminate the horrors that dwelt there.

Aava appears to have control over ice, as it can produce ice pillars. This ability is similar to that of Alsanna who can create ice to protect her, and calm the blizzard that swirls in Eleum Loyce.

Why there were 7 beasts is unclear. It may be that it is simply a thematic connection to the 7 Daughters of Chaos, as the eventual fate of the Burnt Ivory King seems remarkably similar to the Bed of Chaos (the Witch of Izalith). We are even asked to end the life of the Ivory King by Alsanna, which reflects the request of Quelaana for us to kill her mother, the Bed of Chaos.

Lud and Zallen appear to be a saber-tooth tigers. They are two of the seven beasts who served the Ivory King. Each of these beasts was given a specific task. We know of only 3 beasts that remain; Aava, Lud and Zallen. Aava’s task was the guardianship of the King’s beloved child of Dark; Alsanna. Lud and Zallen were tasked with the mercy killing of exiles. From this we know that the beasts were given a broad range of tasks, from protecting life to taking it.

Since the Ivory King was served by 7 of these massive beasts, it indicates he must have been very powerful.

Why there were 7 beasts is unclear. It may be that it is simply a thematic connection to the 7 Daughters of Chaos, as the eventual fate of the Burnt Ivory King seems remarkably similar to the Bed of Chaos (the Witch of Izalith). We are even asked to end the life of the Ivory King by Alsanna, which reflects the request of Quelaana for us to kill her mother; the Bed of Chaos.

Lud and Zallen reside in the Frigid Outskirts of Eleum Loyce, an endless white expanse beset by a near-constant blizzard. These two creatures seem to await any who would wander into their domain. They do not both attack you at once, as they seem confident that Lud, the first to attack you, will be strong enough to deal with any that come their way. That is until you hurt him enough, and Zallen is forced to step in. The two beasts seem to have a bond, as when one is defeated, the other will go into a rage. The appearances of Lud and Zallen are identical to each other, but strikingly different to Aava, the King’s Pet. Lud and Zallen look like they are steeped in Dark. This perhaps explains why they were given a task that drew them so far away from the general populace of Eleum Loyce. While Aava is capable of a sorcery similar to homing crystal soul mass, Lud and Zallen have no such ability, so perhaps some pets were favoured above others or had different abilities.

Lud and Zallen are some of our only clues towards a ‘darker side’ to the Ivory King. They are reportedly under the King’s orders (as each of them was conferred a specific duty), and have been tasked with mercy killing any exiles. The King had a reputation for his great mercy, and we can see here how that idea of mercy has been twisted into cold-blooded murder or capital punishment. Why would the King even need to exile anyone though? Could there have been dissenters amongst the population of Eleum Loyce? Perhaps there were some who did not want to spend the remainder of their life trapped behind the ivory gates, and the King “mercifully” granted their request by expelling them to the frigid, unhospitable and near uninhabitable outskirts of the city. Admittedly, the orders may have come from Alsanna after she inherited the King’s power, possessions and pets, but the idea that Lud and Zallen are “mercy killing” the exiles ties in more closely with what we know of the Ivory King’s lauded mercy.

The Ivory King was once the highest ranking knight from the land of Forossa, famed for its god of war (likely Faraam). He had a reputation for being proud, ever-merciful and for being the first to swing his sword in times of need, be it for his homeland of Forossa or his people. With his magnificent soul, he built Eleum Loyce, a place made to contain the spread of Chaos. He built a Grand Cathedral to appease the raging flame (the Old Chaos) and placed his throne on the mouth of the Old Chaos, serving as the first line of defense. Throughout his life he devoted himself to the protection of the land of Eleum Loyce.

The King kept his countenance from the people by never removing his helmet (which has a face carved into it), he inspired his knights to do the same. The Ivory King kept 7 beasts as pets. They were each conferred a specific duty. Aava was given the task of guarding the Ivory King’s beloved child of Dark; Alsanna. Lud and Zallen were given the task of mercy killing exiles. The King constructed the ramparts, a Garrison House and Golems to guard the ramparts so that nothing would ever escape Eleum Loyce. To this day, the Golem’s continue following the King’s orders.

One day Alsanna, a sister of Dark came to his land seeking the Ivory King. The King offered her his protection. Alsanna was beloved by the King and whether that affection was parental or romantic is unclear. Alsanna found a place at the King’s side. As time wore on the King sensed a degradation in his soul, Alsanna claims that the King was drained of vigor and the Eleum Loyce descriptions states that the King was drained of his strength. He decide to make one last attempt at subduing the Old Chaos. Before leaving, he gifted a sword to Alsanna inscribed with the name of the land: Eleum Loyce. The sword is two intertwining blades, one light, one dark. Whatever else remained of his Kingdom was also gifted to Alsanna. The Ivory King then departed and with his faithful knights entered the Old Chaos, never to be seen again. He was consumed by the Old Chaos and his soul burns in agony, alongside his Charred Loyce Knights.

The Ivory King’s will continues to live on through his Loyce Knights and Alsanna. The Loyce Knights await one who will lead them into the Old Chaos, faithful to the King’s final order to lead the advance on the swollen flame and strike down each malformed terror that arose from Chaos, not hesitating even if it were their own king. Alsanna, devotes herself to a ritual in hopes of appeasing the raging flame, trying to contain the Chaos in honour of the Ivory King. She only wishes that the Ivory King might be freed from the unspeakable Chaos, and that can only happen if he is defeated.

After the Ivory King left, his subjects dutifully awaited his return. Eventually the ivory gates of Eleum Loyce were flung open and the place turned frigid and lifeless. Alsanna was the only one that remained and the name of the Ivory King was lost. This cold brought a final end to the Ivory King’s reign, cold enough to dampen the greatest heat.

While the King’s reputation may have been upstanding with his people, there is evidence that he may not have been as benevolent as commonly believed.
1. He expelled people into the frigid outskirts, where the blizzard always blows. These people were sent there to die, at the hands of the King’s own pets. All that is left of these outskirts are ramshackle houses, but the community may have once thrived, as we see signs of intelligent design because of a Pharros Lockstone contraption. It may be that the expulsion chamber was created after the Ivory King was consumed and that Lud and Zallen follow the orders of Alsanna, who inherited them from the fallen King. This becomes more credible when you consider that the people would likely have turned traitor or tried to flee the land when the King departed, but Alsanna may have wished for them to stay in honor of his wishes.
2. He sealed away weaponry formed by rock, tree and the remnants of a most peculiar soul (the curved nil greatsword). It may be that this sword was forged in the Old Chaos, or perhaps it was something the King feared.
3. He never revealed his face, which may indicate he had something to hide. Though it may have meant he was disfigured, or too humble (which is unlikely considering his known pride).
4. He loved a child of Dark, despite apparently knowing her true nature and gifted her everything.
5. He is repeatedly mentioned to be proud. It was conceit that destroyed the Old Iron King, and perhaps the Ivory King's fate was similar.

Afflicted Graverobber, Ancient Soldier Varg, and Cerah the Old Explorer.

There are no item descriptions or dialogue regarding these three warriors. From their worn items and armor we can guess that they were all grave robbers come to the Sunken Kingdom in search of treasure. We know that the Sunken Kingdom was beset by grave robbers after the fall of the Sunken King, because of numerous clues like the rooms with empty chests and the presence of Flynn's Ring.

The Afflicted Graverobber wears the Alva set and wields Beserker Blades. The Afflicted Graverobber groans with a female voice when hit, so it cannot be Alva. We also know that Alva found enlightenment at Zullie's side, so if we were to encounter Alva, even a hollowed Alva, it would make sense that Zullie would be there too.

Ancient Soldier Varg wears Havel's Set and wields Havel's Dragontooth and Greatshield, items specific to Havel. How he came to be in possession of these items is unclear, but the name we are given (Varg) indicates that this is not Havel, just someone wearing his set and using his items.

Cerah the Old Explorer wears black leather armor, Lucatiel's Mask and the wields Dragonslayer Greatbow with an Estoc. The black leather armor is worn by those with ill intent. How Cerah came to possess a mask just like Lucatiel's is unclear, though it may indicate that the mask was not unique to Lucatiel and may have been a more common issue. The Dragonslayer Greatbow is a weapon wielded by the knights of Gwyn in the Age of Fire. How it came into Cerah's possession is unclear, but it may have been loot, picked up on Cerah the Old Explorer's travels.

Whoever these three are, it is evident that their quest ended in the Sunken Kingdom where they hollowed.

Elana is a child of dark, one of the abyss spawn, a confounded quintessence of Manus and his Dark Soul. Like her sisters; Nashandra, Nadalia and Alsanna, Elana is a fragment of Manus’ emotions. She represents wrath or anger.

There is no mention of Elana outside of her soul and weapon. This is interesting to note in connection with her moniker “the Squalid Queen”. Squalid describes something that is extremely dirty and unpleasant, most often due to poverty or neglect. A Queen is not likely to live in poverty, but perhaps there is no mention of Elana because while she was the Queen, she was largely ignored by the Sunken King who focused on his Eternal Sanctum and the worship of the Slumbering Dragon. Perhaps she grew bitter due to this neglect, and allowed herself to fade into obscurity, all the while plotting vengeance against her King. How was it that Sir Yorgh managed to reach the Dragon’s Rest when all we find is Elana barring our way? When the entrance to the Eternal Sanctum was so heavily guarded, by those who would sacrifice even their physical form to eternally guard the Dragon? Perhaps there was someone on the inside, plotting vengeance upon the Sunken King.

How Elana came to be the Queen is unclear, but it is likely that in a similar way to the other shards of Manus, she took form and came to the Sunken King. She courted this monarch of considerable power, and desired to make the Sunken King’s power her own (see Chime of Want description). Why the shards of Manus seek power is not known. As they are children of Dark, it is likely that they seek to spread the Dark of the Abyss, and therefore extinguish the Light. Or perhaps, like Manus, they are simply driven by powerful emotions and these emotions are best expressed when wielded with great power.

Elana is amassing souls in anticipation of the coming day of vengeance. She is gathering more and more power so that either she can carry out vengeance (as this seems most likely) or so that she can defend herself against vengeance, in a similar way that Alsanna devotes herself to a ritual to escape the coming apocalypse.

We know that Elana sings to the dragon, the same song as the Sanctum Priestesses. The Priestesses also sing to the Dragon, and it is said that they sing to preserve the Dragon’s deep slumber. Elana rests beside the slumbering dragon, Sinh who resides in the Eternal Sanctum built by the Sunken King. She is said to accompany (as in go somewhere with or escort) Sinh. We are encouraged to view the song they sing as a sort of lullaby, but one wonders if a single phrase of their song ever reached the Dragon’s ears. How was it that Sinh became filled with poison? That he continued to fill with poison? From what we have seen of dragons, they do not naturally fill with poison. It therefore may have been that while the priestesses sang, their song cast a spell upon Sinh, and the spell filled Sinh with poison. The Sanctum Priestess tiara says it prevents effects that block spells. Sinh’s soul description says that when Sinh’s rain of death toppled the city, the dragon’s purity was restored. It is interesting that the Dragon had become impure, almost dirty and squalid. Elana and the priestesses’ association with poison indict them further. Elana’s Wrathful Axe creates a cloud of dark poison when its strong attack is used, while when the Priestesses die, they sometimes create pools of dark poison where they fall, almost as if they are filled with it.

The information we know about Elana raises more questions than it answers. How is she amassing souls? From what we know of her, she simply seems to be standing before a shrine dedicated to Sinh, singing her lullaby. How can this then be construed as soul amassing? The only way that souls appear to come to her is when the Player comes to kill her. Are we fulfilling the role of other would-be monarchs who she has slaughtered before?

Who does she wish to gain vengeance upon? Elana is an augur of wrath (extreme anger) and her axe is described as an armament (a military weapon). It may be that she wishes to avenge her dead husband, whose home was invaded and whose life was taken by unnecessary brutality. Or perhaps she wishes to carry out vengeance on a more grand scale, by making everything one with the Dark and extinguishing the would-be monarchs who seek after the Light. Perhaps she is simply driven by a strong emotion, and this emotion does not necessarily have a particular goal, only that it is enacted upon any it can be. The answer is illusive and illumination of her intentions go with her to the grave.

Sinh is an ancient dragon who sleeps in the lowermost depths of Shulva. Long ago the Sunken King erected a grand city and an Eternal Sanctum in which to shelter and worship the magnificence of Sinh. The Sanctum appeared to be a solemn temple, but it was filled with devilish creatures that ensure no trespasser could cause offense. The Sunken King established an order of priestesses who sang to preserve the dragon’s deep slumber and the great dragon continued to sleep soundly. That was until honorable Sir Yorgh, leader of the Drakeblood Knights invaded Shulva and set siege to the Eternal Sanctum seeking the blood of the eternal dragon. The black armored Drakeblood Knights, who came from a land long forgotten, believed that fresh dragon blood was sacrosanct and that by obtaining it they could achieve a true understanding of life, transcending their own banal existence. These knights worshiped the blood of dragons and wore red cloth wrapped around their black armor representing the sacred blood of dragons, as well as wielding greatswords with insignia chiseled upon the hilt symbolizing dragon’s blood. In response to this invasion, the Sunken King’s sanctum knights renounced their own flesh so that they may eternally guard the Sanctum from Sir Yorgh and his Drakeblood Knights, but their efforts were in vain. While Sinh slumbered on, Sir Yorgh disturbed its rest with a single great strike of his spear. With a flash of steel Sir Yorgh’s spear pierced the dragon’s belly and drew the blood he had coveted. But Sinh was not killed, instead, unable to bear it any longer, Sinh spewed forth the poison that had long brewed within him, blanketing the city in a miasmic cloud of death. Sir Yorgh disappeared into the Eternal Sanctum, the Sunken King, defeated by Sir Yorgh, drowned in the poison and the Drakeblood Knights sunk into the Sanctum along with them. The rain of death toppled the city, and shortly after the dragon’s awakening, the towering bulwark, the Eternal Sanctum, crumbled. The knights and soldiers of the Sunken King were asphyxiated by the poison that emitted, their weapons and armor becoming corrupted and corroded by the poison. Even the line of priestesses did not escape the miasma, and their tiaras were warped by poison, perhaps transfiguring them into the monsters we encounter. With the poison exhumed, the dragon’s purity was restored. Sir Yorgh’s spear still jutted out of Sinh’s belly, but the dragon decided to return to its rest. As the Player approaches the city, Sinh is disturbed once more from his slumber and returns to his sanctum, where he awaits his next challenger.

The Fume Knight’s true name is Raime, known as a rebel or traitor to King Vendrick. Raime was an agile swordsman. He and Velstadt, the Royal Aegis were known as the left and right arms of King Vendrick. That was until their wills clashed. Raime and Velstadt fought and Raime was defeated, then deemed a traitor. After his defeat at the hands of Velstadt, Raime came to Brume Tower in search of greater strength. He found it there, but it came not from a regal father figure like King Vendrick, but from a newfound mother, like Nashandra, a child of Dark who haunted Brume Tower and gave Raime true purpose as her champion. Raime had the ability to expunge or remove Nadalia from Brume Tower, but instead he chose to live in the company of Nadalia, Bride of Ash likely because he had become infatuated with her. Raime was imbued with the power of Dark, a part of Nadalia’s soul became housed in his body and she haunted his weapons. Because of this, Raime’s soul fell to darkness. He resides at the bottom of Brume Tower, guarding the room that contains Nadalia’s original body, and the Crown of the Iron King.

Raime’s role prior to leaving for Brume Tower was likely the commander of the King’s military. His sword and armor is remarkably similar to that of the King’s royal soldiers, a group of which seem to have followed Raime into Brume Tower, as their corpses can be found at the base of a destroyed Ashen Idol. As well as this, we know that Raime’s favorite bird was a raven, despite it being despised as an augur of death. If you look closely at Captain Drummond’s helmet you can see that it closely resembles the face of a raven or dark bird. It is therefore likely that Drummond served under Raime in the defense of the Great Fort. Raime’s shield is found within the salamander pit in the Fort. The only way to gain access to this pit is with an Iron Key found in the Iron Keep, the description of which implies that the forces of Drangleic looted the Iron Keep in their construction of the Great Fort (which would have been hurriedly put together or repaired). This may be the reason Raime chose to go to Brume Tower, because he had heard about it or been there before. The only other place Flame Salamanders can be found is within the Memory of the Old Iron King, which indicates that these salamanders may have originated in the Iron Keep. If we are to reconstruct the story, it may be that Raime was defending the Fort against the Giants and lost his shield during a battle, (despite wearing his Baneful Bird Ring), which fell into the pit of the Flame Salamanders. Perhaps he did not retrieve it after this battle because he was hit with a realisation that he took to King Vendrick immediately or was recalled, and Captain Drummond was commanded to take charge of the Fort.

Why Raime and Velstadt disputed is not known for certain. It may be that news came to Raime on the front line that the King had abdicated the throne and left his castle. Raime, having seen the war that was tearing Drangleic apart, could not stand by and let his King simply abandon the effort. However, by this point King Vendrick had exhausted every effort he could muster in his attempts to overcome the curse, without success. Raime may have confronted Vendrick and Velstadt, but Vendrick tenaciously held on to his decision. Velstadt, always loyal to his King, disputed with Raime and the argument became so fierce the two ended up fighting. Raime, then an agile swordsman, was defeated by Velstadt, a hulking cleric and the King made good on his word to leave. This may not be what actually happened, but would explain why Raime was deemed a traitor, never returned to his King and why he was not found on the front lines.

Sir Alonne came from an eastern land and chose to serve a little-known and unestablished lord. He became the lord’s most trusted knight and helped him become the Old Iron King. Sir Alonne was a master of the quick-draw and in obeisance to his new lord, Sir Alonne trained the Old Iron King’s men in the sword.

After establishing himself, the Old Iron King descended into absolute depravity. Once, the Old King could have unraveled the greater mysteries, with the aid of his great soul, but he was led astray by his enormous fortune, becoming nothing more than a vulgar hedonist.

At the end of the very peak of his sire’s rule, Sir Alonne disappeared. He was said to have deserted his liege after bearing witness to his descent into depravity, setting out again in search of lands yet unknown. But the loyalty of the Alonne Knight was the stuff of legends. These men, trained by Sir Alonne were renowned for their loyalty, and yet their leader deserted his liege. These two facts do not seem to fit. The Player encounters Sir Alonne in the Memory of the Old Iron King. We know that acts carried out in Memories do affect the history of reality, as seen in the battle against the Giant Lord. If this is the case, then when the Player kills Sir Alonne, they bring an end to his story, and the rumors we read of were simply started as an explanation for this seemingly random event.

After his departure, whether by the Player’s hands or because he truly did set out again, the Old Iron King bequeathed Sir Alonne’s name to his iron warriors. This seems very much like a gesture to a deceased friend, who had helped him become what he was, than the honoring of a traitor who had abandoned the Old Iron King right when he needed him again.

Sir Alonne was a warrior who valued honor greatly. He will bow to the Player before they fight, and if defeated by the Player without taking damage, Sir Alonne will perform Seppuku (ritual suicide).

It is likely that by killing Sir Alonne, we set into motion the events that would eventually end the Old Iron King’s reign. The Undead Curse was spreading throughout his Kingdom and he was in desperate need of guidance, yet his most trusted knight had just departed. The Old Iron King may have turned to his loyal pyromancer; Eygil. Together they may have created the Smelter Demon, perhaps to replace Sir Alonne by his side. But the Demon turned on the Old Iron King and cut him down. His body sunk belong and met the one whose name must not be repeated, becoming ichorous earth. His body began to breed lava and eventually it subsumed the Iron Keep.