The Necromancy Mastery ability "Control Undead" lets you take control of most Undead mobs, at 2 Control Slots per. They can't be stabled. Skeletal Dragons are the most powerful undead that can be controlled usually, and are equivalent to Shadow Wyrms. Controlled Undead gain Life Leech that is scaled based on your real Necro/SS skills, so undead mobs that hit hard with their melee (like the Skeletal Dragons) benefit the most from it.