A Unicorn Horn (ancient horn, if unidentified) is a rare consumable object that decreases the sanity of everyone non-hostile in range by 50.
One may obtain two unicorn horns by completing the Wife Collector quest. There are also always a permanent unicorn horn outside the mages' guild in Lumiest and on the floor in the Thieves' Guild, both of which can be stolen or used on the spot. There is a unicorn horn you can pick up on the first level of the Pyramid. Souvenir vendors in Noyel during Festival of Jure and at Larna will sometimes sell them.
A cursed unicorn horn seems to have the same effect as a non-cursed one.
It weighs 2.0 stones.