The minions of Summoner's Rift are descendants of a tribe of highly-intelligent trolls who spent centuries studying the energies of a shattered boundary into another world. These giant, lumbering creatures discovered arcane secrets that promised the ability to change the very concept of reality on Runeterra. In order to ensure they could harvest the raw magical energy needed to infuse the runes that were the basis of their studies, the tribe's greatest mages combined their powers and collected them in a giant crystal, intending it to be a versatile power battery to be unleashed when needed. The crystal, to their disbelief, absorbed all magical power that surrounded it, including that of the trolls.
One amongst their number, a troll named Lari, wailed against the injustice of the event, and declared he would destroy the crystal that had grown into the nexus of all local magical power. By doing so, he posited, he would restore his people to their former glory. Though their magics had been utterly extinguished, Lari knew his tribe had one remaining facet that could crack the nexus: their monumentally thick skulls. With this in mind, Lari ordered all remaining trolls to take a long run up and smash their heads into the crystal.
Initial efforts proved futile, and messy, but Lari persevered. Eventually, he hit upon a plan to repeatedly smack the trolls' skulls in quick succession into the nexus crystal to gradually weaken it. He commanded the entire remaining populace to line up in single file and race towards the tough, sharp, and often blood-smeared crystal.
Once the first pile of concussed and dead trolls was moved out of the way, Lari himself led the charge for the next wave of brave volunteers, desperate to recover their magic. Lari was unable to carry on his quest after that day (because his calendar was suddenly filled with meetings about drooling into his own lap), so he retreated to his hut, scribbling incomprehensible gibberish into a giant tome.
Over several millennia, which seemed to go on for eternity, his tribe continued Lari's work. Thanks to the power of evolution, the trolls eventually adapted to their new way of life by becoming greatly diminished in size and developing huge, plated foreheads. To hide their diminutive ugliness, and to pay homage to their heritage as former mages, the trolls adopted the practice of wearing obviously oversized robes.
To this very day, each morning the minions of Lari wake up, wave at the statues of their leader, and dash along the lanes plowed by centuries of small feet pitifully running head first into a glowing, apparently impenetrable crystal. Many historians have difficulty believing that billions have died of awful injuries and general heartache due to the raving madness of one giant idiot named Lari, a troll who spent his last days slopping nonsense into a book for others to read. As has been said by many witnesses to the appalling daily ritual (shouted loudly over the sound of skulls repeatedly bouncing off a large gem), there's no bigger troll than a Vietnamese noodle-shop worker.