Boss Drops
The Warrior Soul boss only drops 1 item in Universal Piece. This boss has a 50% chance to drop the Soul of the Warrior item. You will need to be in the top 3 of the damage dealers in order to receive the rewards chest.
When you get the Soul of the Warrior item you can go and talk to the Hell Power NPC. You can exchange the Soul of the Warrior and 50 gems and 50 diamonds for Armament Force or Instinct Force.
Armament Force will buff your melee attacks while the Instinct Force will give you 10 auto dodges.
You can get both of these Forces if you farm this boss and get 2 Souls of the Warrior.