An event has left the Big Bad or other major antagonist severely injured or drained of power. It doesn't have to be a machine; it can be the inability to survive without attatching to another body.
This can be a way of creating sympathy for the villain, showing that they are weak or dependant. The weakness can be exploited by someone attacking them -- 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. Frequently, the very thing that made the Dark Lord this way was done by the protagonist, sometimes in a battle that didn't succeed in killing the enemy but did injure him. This can be a sign that the hero wasn't ready to fight the villain, that he did so rashly, etc..
This may result in a Dragon-in-Chief. Not to be confused with Evil Cripple. See also Man in the Machine, Brain in a Jar and No Immortal Inertia.
In The Vision of Escaflowne, Big Bad Dornkirk (actually Isaac Newton transported to Gaia) is perpetually hooked up to a massive life-support machine keeping him alive well past when he should have died of old age. In the last episode, he rips free of it and lets Folken kill him to set in motion his Thanatos Gambit.
Taopaipai and Freezer from Dragon Ball Z were both rebuilt as half cyborg, after Goku severely damaged them, in order to survive (both had several parts of their bodies replaced by mechanical components because of the damage sustained).
Full Metal Panic! The Second Raid: Gauron has been reduced to lying in a bed tied to a machine after he was defeated in the previous season, but still manages to ruin everyone's day.
After having his life saved by Heaven Canceller, Alistair Crowley from A Certain Magical Index has only once ever been seen someplace other than floating upside down in his tube. It's debatable if hew as actually physically present at that one time.
Kagemaru, the Big Bad of Yu-Gi-Oh! GX 's Scared Beast/Phantom Demon arc, is at first confined to a full-body life support mechanism.
In Naruto, after getting his arms sealed by the Third Hokage Orochimaru becomes this, and eventually he gets progressively weaker and more sickly when he has not transferred his mind to a new body every three years. Sasuke takes advantage of this, and defeats him in his sickly state.
Star Wars: Darth Vader is probably the Trope Codifier. He can't live for more than a few minutes outside his black armor. He has bionic limbs, and his breathing is also reliant on robotics. This is from a time when Obi-wan Kenobi dueled with him when Vader was trying to take over the galaxy with Emperor Palpatine. Obi-wan wasn't able to kill Vader because they had once been like brothers, but he left him burning and without several limbs. Vader barely survived, and was taken by the Emperor and given his life-support system.
Overdog in Space Hunter: Adventures in the Forbidden Zone is a cyborg who is linked to a life-support system that lets him suck the life force out of his victims and infuse it into his body. The apparatus also serves as a means of moving around in his lair as he has no lower body and instead is connected to a crane-like structure.
Example from the Teen Titans film Trouble In Tokyo where he basically creates an ink army: this. this◊. He gets defeated and the Bigger Bad takes over.
Voldemort lives off Quirrel in Harry Potter and the Philosopher's Stone. Since he was nearly destroyed, he is too weak to survive alone and even has to have unicorn blood to stay alive. This means he's weak enough that he can barely talk; when Quirrel is killed, he flees like a ghost.
And even more repulsively when he's reduced to a helpless fetus that relies on Wormtail's care-giving and lives on snake-blood. Even then, he's still got an attitude.
Mason Verger in the film and book Hannibal is a depraved child molester who was attacked by Hannibal Lecter but survived as a disfigured paraplegic, and therefore utterly depends on his caretaker to carry out his scheme of vengeance against Lecter. As a result of what Lector did to him at their last meeting, he is now on a respirator, bedridden and relies on a machine to moisten his lidless right eye. In both versions, Lecter uses a Hannibal Lecture to convince his caretaker to kill the helpless Verger in retaliation for years of abuse.
In Otherland, Jongleur lives off of life support, due to being the oldest man alive and pretty much has to live in his program since his body is failing.
Ah Ling/Hendrik van Eeden/Tzaddik from the Sally Lockhart series by Philip Pullman is this. Although Sally shoots him in The Ruby In The Smoke, we find out in The Tiger In The Well that he survived, but her bullet had pierced his spinal cord, leaving him paralyzed.
All the Daleks. Inside those fearsome 'travel machines', they're just these helpless little squidlike things.
Davros, the creator and for a while the ruler of the Daleks, kept alive by a life-support system inside his high-tech wheelchair which can be turned off by pressing a button.
Dr. Lumic, the creator of the new series' Cybermen.
Max Capricorn in the Christmas special "Voyage of the Damned".
Lady Cassandra O'Brien.Δ17 considers herself to be the last human being, but has undergone 708 cosmetic operations and is now nothing more than skin with a face, stretched over a screen, with her brain in a jar underneath.
Scorpius in Farscape. A Sebacean-Scarran hybrid, his body chemistry was literally at war with itself and so he required an extensive cybernetic cooling system just to maintain a functional condition. Enemies who learned of this weakness frequently tried to take advantage of it to kill or disable him.
Anubis in Stargate SG-1. Due to having ascended and then being kicked partway back down to our plane of existence, he currently exists as an Energy Being that requires some form of assistance in order to interact with our world. From season 5 until "Lost City", that assistence was a force-field suit. After its destruction, he had to Body Surf between hosts whose bodies reacted to him as if he were a disease.
The Joker from Batman: Arkham City has to be kept on life support because he's dying from Titan poisoning. The Titan serum that mutated him and gave him superhuman strength at the end of Arkham Asylum actually had unforseen side-effects; he has a sickness that is slowly killing him.
Umbrella president Oswell E. Spencer is reduced to being confined to wheelchair and life support system three years before Resident Evil 5 due to his old age. He expects Alex Wesker to reverse this condition by engineering an Immortality virus, although Albert Wesker kills him to become the Big Bad after Spencer reveals that Wesker was one of many Unwitting Pawns for Umbrella known as "Wesker children" in a project done to create the perfect breed of humans.
Extreme inversion: Mother Brain, big bad from the Metroid series is a giant brain in a highly impact resistant glass casing. After breaking through the shield you still have to fire multiple missiles(regular beams do nothing) directly at her bare grey matter in order to finish her off proving that the transparent casing is clearly for protective reasons only.
In Team Fortress 2, the teams' owners, Blutarch and Redmond Mann, are among the closest things to evil overlords in this universe. Blutarch is on a life support/extender machine just so he can live longer than his brother Redmond, who is also stated to be on one. Since the machines are about 100 years old, they don't run pergfectly, and thus the brothers die every now and then.
Count Tarrorviene of Annyseed is hooked up to a blood machine which he is dependent on for survival. He also keeps his most capable servant at his side all the time, rather than sending her out to accomplish tasks that are difficult for his lesser servants, because he's incapable of defending himself and fears for his safety.
Five hats means that five tropers think it is ready to publish.
You are saying that you think this draft is ready to be published. That means the description is not ambiguous,
it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
Is that what you meant to do?
You are saying this draft has a ready-to-publish hat it does not deserve and you are taking it back.