It came to pass on one of these adventures that the unicorn was killed; this is not unusual, either, for while unicorns are long-lived, they are still mortal. The unicorn's companion managed, with some difficulty, to survive the circumstances of the unicorn's death, and, after some considerable effort, secured the unicorn's body. She removed the unicorn's horn-- a task which can not be accomplished by brute force, but is easy for one who knew the unicorn in life, and knows the proper spell-- and then burned the rest of the body. She took the horn-- the alicorn-- and sought out a Great Wizard, for she did not have enough magic for the task she needed done.
Now, alicorns are wonderful things, on many levels. The material is lighter than water, stronger and more flexible than steel, and nearly as hard as diamond. Of course, this means that it can not be worked by any normal tools, but there are alternatives. When a unicorn dies (and there is no other way for an alicorn which is not attached to a unicorn to come into existence), its spirit takes up residence in the alicorn. So while the alicorn is not really alive, neither is it truly unliving, and certain spells which shape living flesh can be used, with the consent of the unicorn's spirit, to shape an alicorn into something somewhat more useful than a simple horn. It was for just this reason that the unicorn's sorceress sought out a Great Wizard.
In due course, the sorceress found a Great Wizard, and the alicorn was shaped into a dagger, an athame. The alicorn athame was so beautiful that the Great Wizard wanted it for himself, but the sorceress would not part with it, and went on her way.
The sorceress wandered the wide world, and had many more adventures. The spirit of the unicorn who had been her master was with her through all of them, aiding her with advice and with various magics which were intrinsic to an alicorn athame. The greatest of these was the ability to bolster any attack made with the alicorn with the strength of her own personality.
Over time, the Great Wizard came to regret allowing the sorceress to escape with the alicorn, and he began to plot to get it for himself. He sent thieves to steal the alicorn, but they failed; he sent assassins to kill the sorceress and THEN steal the alicorn, and they failed as well. Eventually, he became angry, confronted the sorceress, and blasted her to ash. And then he took the alicorn home with him.
For a long time, the wizard attempted to communicate to the spirit of the unicorn within the alicorn, for the powers of the alicorn can not be used without the consent of the unicorn within. The unicorn ignored the wizard, and waited, and hated.
Eventually, the wizard gave up. He was Great Wizard; he had a dragon bone staff; he did not NEED the powers of the alicorn. And it was still beautiful, and rare, and very, very sharp. He still had occasional use for a dagger, and if nothing else he could use it to slice sausage. As he thought these things, he tested the edge of the alicorn with his thumb, and because he was not accustomed to handling weapons, he cut himself.
The cut released a single drop of blood, a tiny drop, but it was enough. The power of the alicorn flared, and turned the force of the wizard's personality (which was significant, for he was a Great Wizard) against the wizard's body (which was relatively old and frail). The wizard's heart exploded, and he fell dead. The wizard's spirit retreated into his dragon bone staff; the alicorn fell to the floor, bounced, and rolled to a stop mere inches from the dragon bone staff.
They remained that way for many years, inches away from each other, two angry ghosts in their respective prisons, sharing only their hatred for each other.