Horn: 2.5' with razor sharp ridges that will cut if stroked from tip to base. A pale, clear blue that always seems to glow faintly. The horn is hollow and capable of producing sound. Horn is capable of healing, purifying water, poisons, and negating magic. Healing only occurs to a small radius from the point of contact, about five to six inches.
Due to her nature most magics won't work on her at all. Only the most powerful spells or the effects of another unicorn are likely to directly affect her. There are always exceptions though. She just hasn't met any.
There is a legend about unicorns that if one is pure in heart and mind that the unicorn's horn will heal the person entirely if the horn pierces the heart. Variations of this involve the person becoming immortal or becoming another unicorn. Even the unicorns are uncertain of the result and are therefore generally unwilling to try it unless the person is already dying.