All instances of Devour (whether prompted by an Unleash or a nemesis card) are performed by Prince of Gluttons. Any time a Devour would be performed on an empty supply pile, Gravehold suffers 2 damage per card.
Example: If there are no gem piles remaining and Godfeeders is in play (Persistent: Gravehold suffers 3 damage. Devour a gem from the most expensive gem supply pile.), Gravehold takes 5 damage (3 damage plus 2 damage from the attempted Devour) when evaluating its persistent effect.
