When you field your first Pit Fiend each turn, you may move 1 NPC die from your reserve pool to your used pile to draw 3 dice. Roll any evil character dice drawn this way...
This is a great example of the clever theme integration into this game. The Pit Fiend can sacrifice one of your NPCs in order to summon up to 3 more evil characters. That's cool.
(Yes, I realize that the Pit Fiend's abilities in D&D don't require human sacrifice in order to summon other evil creatures. The fact that it requires it here most likely helps to balance the mechanic while still keeping with the infernal coolness.)