Warrior Creed is a silver chest drop from Coop 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 Hard stages, the rate is low enough to not ever expect one to drop from Hard. Most players farm Corridor of Puzzles or Lost in the Dark for their relative easiness when collecting Warrior Creeds.