Warrior Creed is a silver chest drop from Co-op Quests.
|Unlock Ninja or Samurai||1|
|Unlock Sword Master or Assassin||5|
|Forge Class Champion Weapons||10|
|Rebuild Class Champion Weapons||25|
While there is a possibility of it dropping from any stage, the rate is very low outside of extra coop quest. Most players farm Corridor of Puzzles or Lost in the Dark for their relative easiness when collecting Warrior Creeds.