Sylvanas' "Possession" on level 10:
Force an enemy Minion to fight for you. It gains 30% Attack Damage and Health. Cooldown increased to 20 seconds when used on Catapults.
... and on level 20 "Dark Lady's Call":
Increases the Attack Damage and Health bonus to 30% and allows Possession of enemy Mercenaries with a 20 second cooldown.
... seems not that useful unless it works on boss. Does it?