Aspect of Lies, Belial's Assassin, is a unique Enslaved Nightmare found in Silver Spire Level 1 in 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, Aspect of Lies has Mortar, Plagued and Teleporter affixes, and can summon demons at its side instantly and with reduced cooldown.