An
integral part of any
NetHack adventurer's
inventory. Without one, you will probably
die.
That said, a unicorn horn is the magic fix-all for most NetHack ailments, in particular food poisoning, hallucination, blindness, confusion, inebriation, stun, vomiting, or lost ability score points. Applying the unicorn horn usually works, but not always -- the chance is about 1/5 (uncursed) or 1/3 (blessed) of fixing a single problem. It will occasionally fix multiple troubles at the same time. Not only that, but it can be used as many times as you want -- no charges, risk of explosion, or bizarre side-effects! A true panacea.
A cursed unicorn horn gives you one of the ailments named above, or (much more commonly) exacerbates the ailment you were trying to cure. You will die of food poisoning much faster if your unicorn horn was cursed.
Dipping a unicorn horn into a potion of blindness, potion of confusion, or potion of hallucination will turn it into uncursed water. Dipping it into a potion of sickness produces uncursed fruit juice.
Unicorn horns also make decent two-handed weapons, comparable to a two-handed sword. You should always possess multiple blessed unicorn horns lest you be waylaid by Demogorgon -- you're a goner for sure if it catches you without one.