Sacrifice

Drop a corpse on an altar and use the extended command #offer. If the corpse is fresh enough (<50 turns) it will be sacrificed. Depending on your alignment and the alignment of the altar, good or bad things might happen. Sacrificing on an altar of another alignment has a chance of converting the altar to your alignment, or converting you to the altar's alignment (rendering the game unwinnable).

Special things to sacrifice (or not)
Do not sacrifice corpses of your own race if you don't want to anger your god. Do not sacrifice (former) pets. Do not sacrifice unicorns unless you really know what you're doing.