Unicorn's Horn

Source

 * Can loot one from the Unicorn in Fey Glade if you start a fight against the fey mobs. (the unicorn appears to run away without even rolling initiative. Not sure if this information is still accurate).
 * One can be found together with some scrolls in the room section (room to the right at the end of the corridor) - western part of the Abandoned Keep in a trapped Trickery 36 and locked Trickery 22 chest during the main quest Betrayer's Flight.
 * A Random Encounter in the Narlmarches gives you the opportunity to attack 4 Unicorns. If you kill all 4, a Hamadryad will appear to attack you as well.