Nighthorn

Nighthorn is an intelligent lawful artifact unicorn horn obtained at the end of the Lawful Quest in SLASH'EM. It inflicts no extra damage, but provides reflection when wielded.

Generation
Nighthorn will never be randomly generated, but is instead found upon killing Nightmare on the single-level Lawful Quest, a portal to which is found between levels 15 and 19. It will only be found if the manner of Nightmare's death would have otherwise left a corpse - disintegrating her or digesting her will not get you Nighthorn. Note that the Key of Law will still be found, though. The dropped Nighthorn will always be cursed and +0, so be very careful before wielding or applying it!

Usefulness
Overall, Nighthorn is in general more useful than the Neutral Quest artifact, the Eye of the Beholder, but considerably less so than the chaotic one, the Hand of Vecna.

As a unicorn horn
To neutrals or chaotics, this is likely to be very nearly useless. It provides no benefit from being carried, and the 8d10 blasting damage it will inflict every time it is used makes it difficult, if not impossible, to use as a unicorn horn. For lawful characters, however, an intelligent unicorn horn is quite useful, as it will resist curses 80% of the time. However, it is certainly not a necessity, and probably not worth using more scrolls of enchant weapon if you already have a well-enchanted unicorn horn.

As a weapon
A mid-game character might find this a useful weapon, as reflection becomes a larger and larger priority as the game goes on. Lawfuls in particular might benefit from having it function as both a unicorn horn and a weapon, since enchanting helps both of these functions. However, even a neutral or chaotic who really needs reflection and is not restricted in the unicorn horn skill might find it handy. Unfortunately, Nighthorn simply does not do enough damage to be a contender in SLASH'EM's later game. It is also two-handed; while it does provide the same property that a shield of reflection provides, it does not provide the same 6 or 7 AC bonus of a well-enchanted shield.