sacrificing in NetHack

• In Gehennom - completely useless. You will only anger Moloch.
• On another god's altar when your alignment is negative - you will convert yourself. If this happens before the Quest it will render your game unwinnable.
• Sacrificing former pets - no longer a good idea in 3.3.1.
• Sacrificing your own race - only a good idea if chaotic or polymorphed into a demon. Even then it won't ever get you an artifact (IIRC).
• Sacrificing unicorns - this can be good or bad, depending on the alignments of you, the unicorn and the altar.
  • Sacrificing a unicorn on an altar of its own alignment is an insult to the altar's god, who will hurl lightning and disintegration at you.
  • Sacrificing a unicorn of your own alignment on an altar of another alignment will convert you instantly - as mentioned above, this is a bad thing.
  • Sacrificing a unicorn of another alignment on your own altar is good and carries an extra bonus.
  • Any other combination (you, altar and unicorn of three different alignments) is a normal sacrifice.

If you sacrifice on another god's altar (not in Gehennom), and your alignment is positive, it is an attempt to convert the altar. The probability of success depends only on your experience level. A successful attempt increases your luck by one; a failed attempt decreases it by one.

If you sacrifice on your own god's altar, the consequences depend on your general standing with your god.

• If your god is angry with you, all sacrifices are interpreted as an attempt to mollify him/her. Only tough monsters are any use for this purpose. If the monster wasn't tough enough, you have a feeling of inadequacy. Otherwise, your god may seem "slightly mollified" if they are still angry, or "mollified" if they are no longer angry. Mollifying your god also fixes negative luck.
• If your prayer timeout is above zero (you can't pray yet), sacrificing will reduce it. Tougher monsters are better for this purpose, but anything will do. If you still can't pray, you will have a "hopeful feeling". If you are now able to pray, you will have a "feeling of reconciliation" and negative luck will be fixed.
• Otherwise, if you are at least experience level 3, you have a chance of gaining an artifact. The chance is independent of the toughness of the sacrifice and of any properties of your character, but goes down as more artifacts are created.
• If you could have gained an artifact but didn't, your luck may go up - tough monsters give bigger luck boosts. The message associated with an increase in luck is "You glimpse a four-leaved clover at your feet." If you don't get this message after sacrificing a tough monster, your luck is probably maxed out.