Nightmare

Nightmare is a monster added in SLASH'EM. She is the Lawful quest nemesis, and will leave the Key of law when killed and Nighthorn if killed in a way that would leave a corpse - Nightmare will never leave a corpse, instead simply producing Nighthorn and the message "All that remains is her horn...". Like all alignment quest nemesises, Nightmare is covetous and will teleport to your position once awakened. Nighthorn is not strictly a unicorn, and thus will not accept gems nor use her horn to heal herself.