From D&D Wiki
A unicorn is a rare, special creature. They are thought to be one-of-a-kind and unisex/ungendered. That means they do not reproduce, so their origin is magical in nature and to kill one is a great evil.
A unicorn is so rarely found in the wild, that they are considered extinct by most commoners. Only the wizards seem to believe in them. The unicorn lives in a magical universe and is very attuned to avoid dangers like evil-aligned areas. Similarly, they avoid evil aligned people. If you are to see one you must perfect yourself, or be so aligned with their environment that you can enter unnoticed by the keenest perception PER 20.
The alignment generally needed to approach a unicorn is a perfect +1000., otherwise they generally don’t bother with wondering about you at all. Dark times, outside the purity of the feywild, will make a unicorn bolt away (or charge you with its horn). This is a bit misleading, though, because if a alignment 500 character kills a good-aligned creature that may be only 100 pts of power, they don’t just become 400 aligned but now have karma at the point of their alignment.
- Without any special arcane knowledge of the foregoing (like countering dark magic), the horn has the ability to confer wishes at 50% likelihood (or power) as a wand of wishing, but conferring dark versions of whatever is wished for. The remaining time nothing is granted at all and the universe just gets sadder (a few more dragons have presence in the realms). (Even a wand of wishing fails, now and again, doesn't it?). A perfect wand of wishing by the way, requires the realms to be held by a wizard, and gives you exactly what you wish for.
- This chance of getting a wish is entirely alignment-dependent and requires the universe to be in alignment enough also (based on the state of the realms and your attention on the object wished for).
- A unicorn horn can be used to purify water if it is contaminated (turning it into the 800 level water at 1ft3 at a time). One can powder the unicorn horn and drink it in a "tea" and be healed of most organic diseases. This practice is frowned upon as it encourages poaching of any unicorns. Try not to....
- There is a unicorn in the Ravenloft in the small, unmarked clearing between the entrance to the realm and the castle (underneath).
- People often picture them as horses, but this is probably whimsy. More likely, it is the perfected version of the goat. Most goat carry the hate of the animal kingdom towards man which you can see in how they look at you. The hate is stored in their horns. But when they channel it though perfect environments, their normal two horns become one.