Deathjacks

Deathjacks are a rare form of undead that take over the cortexes of destroyed steamjacks and begin to animate their metal bodies. Even though the arcane boilers lie cold and inactive, the psychic energy of the undead's spirits power the hydraulic mechanisms to move the body.
Most newly-animated deathjacks are little more than twitching wrecks. Eventually, it appears as though the spirits "figure out" how to move the steamjack bodies. Ancient deathjacks, some as old as a century, have mastered the controls and can expertly attack with weapons or even move faster than the steamjack originally could. Deathjacks are incredibly durable and nearly impossible to take down, as damage to the iron shells does little to harm the animating spirit controlling it. Usually, it is better to flee and hope it forgets about you, or try to disable its legs or weapons and then flee.
While most deathjacks are naturally occurring, some have figured out how to purposely produce deathjacks, such as at Greybone Fortress. These deathjacks are more purposely bonded to steamjack cortexes and are more capable of intelligent thought and control. They also can make use of the existing steam turbines for even more power and speed.