Mobs must be activated by getting within melee range, but they remain rooted and undamageable. Then you must clear the room of spectral lords and acolytes. One of them will un-root and attack you first, when it is down, clear the room again, the other will un-root. Each debuffs you when they un-root, and if you want, you can release it by clearing spectral lords or acolytes depending on which debuff you have.
