Devour lets one creature consume another (or a Devour item) and take what was worth keeping: the fodder's XP always transfers in full, grade and mutant status transfer only within the same line, and the fodder is gone for good.
1Open the consumer's sheet
Open a creature from the Soul Bag or the Inventory. DEVOUR sits in its header beside LOCK and DISMANTLE - this creature is the one that eats. It opens at floor 12.
2Pick the fodder
The consumer stands on the left. Click SELECT FODDER to choose what it eats: another creature, or a Devour item from the Soul Bag.
3Choose from the list
Every creature you could feed is listed with its grade and level; the ones in your party are greyed EQUIPPED and cannot be eaten. Click one to pick it.
4Confirm
With both sides filled, the panel says what will transfer. Click DEVOUR to finish - the fodder is destroyed permanently, so read the line first.
Screenshots are taken from the live game by tools/tutorial_shots_b.py (dfmc-client) and regenerated after interface changes. The rules behind each screen: system-devour.