Dark Lord on Life Support

An event has left a villain severely injured or drained of power. So, his minions hook him up to a device that will keep him alive for the time being. It doesn't have to be a machine; it can be the inability to survive without attaching to another body.

This can be a way of creating sympathy for the villain by showing that he is weak or dependent. The weakness can be exploited by someone attacking him. The Evil Overlord may not have full strength because of what's happened to him or may be stuck in one place.

Dependance means that The Hero can destroy the support system to destroy his enemy (although there might be moral issues with dealing the killing blow to a villain who can't fight back).

Frequently, the very thing that made the Dark Lord this way was done by the hero, sometimes in a battle that didn't succeed in completely killing the villain. This can be a sign that the hero wasn't ready to fight the villain, that he did so rashly, etc.

Please note that it is possible for said Dark Lord to recover from this, maybe even come back stronger than ever.

Aesthetically, this trope associates the villain with illness and death, which in turn makes him seem creepier than he would be otherwise.

Examples

 * Bane in The Dark Knight Rises wears a metallic mask which constantly supplies him with anesthetic gas to keep the crippling pain from his old injuries at bay. It's frightening to say the least. Unlike many examples, unless it is damaged, the apparatus does little to hamper his combat ability. As a matter of fact, the cocktail of drugs dispensed by the mask suppresses his sense of pain, which contributes to his durability.
 * Jigsaw in the second through fourth Saw movies. His cancer has finally reduced him to the point that he's permanently bed ridden and relying on his assistants to carry out his plans.