Aspect of Lies, Belial's Assassin is a unique Enslaved Nightmare found in Silver Spire Level 1 in Act IV of Diablo III.
In Campaign Mode, it appears in form of Mira Eamon's ghost, attacking after players talk to it. Killing it is not required for the quest.
In combat, the Aspect of Lies has the Mortar, Plagued and Teleporter traits, and can summon demons at its side instantly and with reduced cooldown.
A bounty may be offered to kill the Aspect, in which case the secondary objective is to kill 75 enemies.