• 0.[CotG] Chronicle of the Godslayer : The basic game
  
• 1.[RotF] Return of the Fallen
  
• 2.[SoS] Storm of Souls
  
• 3.[IH] Immortal Heroes
  
• 4.[RoV] Rise of Vigil
  
• 5.[DU] Darkness Unleashed
  
• 6.[RU] Realms Unraveled
  
• 7.[DoC] Dawn of Champions
  
• 8.[DS] Dreamscape
  
• 9.[WoS] War of Shadows
  
• 10.[GotE] Gift of the Elements
  
• 11.[VotA] Valley of Ancients
  
• 12.[DLRM] Delirium
  
• 13.[DLV] Deliverance

CotG, the basic game and "10th", in the below picture, are basically the same. "10th" means 10 years aniversally, which has updated arts than the original one, CotG.



Since some sets share same mechanics, there are some combinations, which work together.

• 0+1: CotG + RotF: Chronicles of the Godslayer & Return of the Fallen
  
• 2+3: SoS + IH: Storm of Souls & Immortal Heroes - for the Event cards
  
• 4+5: RoV + DU: Rise of Vigil & Darkness Unleashed - for Treasure and Transform
  
• 6+7: RU + DoC: Realms Unraveled & Dawn of Champions - for multi-facition heros, constructs, and synergies between them
  
• 8+12+13: DS + DLRM + DLV: Dreamscape & Delirium & Deliverance [No Recommended for Balance] - for Insight Mechanic

0+1, 2+3, 4+5 are meant to be played together, by design. You can also just play with only one set.
After the 6th set, each set are meant to be played alone, by design. You can certainly mix them, but the result might not be that good.
6+7 is a good result from mixing.

• Good: 2+3; 8; 0+1
  
• Mid: 12; 13; 9; 11
  
• Bad: 4+5; 10; 6; 7: 6+7

The above recommendation is purely subjective though.

Newbie: 0+1➡2+3➡8
Veteran: 12➡13➡9➡11

• 2 Player: Recommended;
  
• 3 Players: Highly Recommended;
  
• 4 and more: Not Recommended.

When playing offline against the AI, there are two level of AI to choose from.
Red is level 2, while white and green is level 1 AI.



As I said above, you do not need the CotG, the basic game, to play any set. Any set can be played alone. This might not be abvious, since the in game menu has CotG or "10th" selected as default.
Just click on any set and click back on CotG and then you would deselect CotG.