A darkenbeast, also called a death horror, was a magically created beast.
The darkenbeast attacked its target with a combination of fangs and claws. They were immune to mind or monster control spells. It would obey the telepathic commands of the caster, who gave limited orders based on mental images. The darkenbeast would hunt the target until the target was killed, or daylight broke the spell.
It was a magical creature with limited existence. A darkenbeast was born when a mage cast a create darkenbeast spell on one or more animals. This spell could only be cast in darkness. The target animal(s) would remain a darkenbeast for the duration of the spell, and would revert to its normal form, whether living or dead, when exposed to daylight. Mages in the Underdark were known to keep packs of darkenbeasts permanently.