Absolute Power Exchange
One character gives another character the power to totally wreck them, if they chose to.
Every human being requires a certain level of power to function. In order for politicians and cops to do their jobs properly, they have to be given certain levels of authority that, if abused, would have dire consequences. In more mundane circumstances, a boss has a large amount of authority over their subordinates and a teacher has the same over their students. This is why punishment is severe when said person in a position of trust abuses it, especially in cases of abuse or harassment. However, this trope is about people who are otherwise peers--friends, coworkers, siblings, neighbors, significant others--until one of them decides to give the other some type of power (whether a physical object, secrets, or money) that could completely screw them over if abused. It may be a Superhero that is a Person of Mass Destruction that lets the Five-Man Band know what their Weaksauce Weakness is, a Gentleman Thief who lets his Morality Pet know where he stashed his biggest heist, or a shy teenage girl letting her favorite, kind-hearted teacher know her biggest, darkest secret. Either way, they have completely put their future into this person's hands. The reasons for this are numerous; it's obviously a sign of trust, and the person giving away the power may want to indicate how much they believe in the other party. Another reason is to keep themselves honest: if they ever step out of line, make a Heel–Face Turn, become possessed or wind up Brainwashed and Crazy, they trust this person's judgment enough to do what needs to be done. These are only a few examples--the are a near infinite number of reasons why this can be done, including just for the fun of it. One type of this is telling someone else your Secret Identity or True Name.
- Superman, in the post-Crisis reality, handed Batman a kryptonite ring that he'd taken away from Lex Luthor. The reason was that Superman was Genre Savvy enough to know that if he ever found himself on the wrong side (for whatever reason), it would take someone like the Dark Knight to take him down. Also, it was a sign of just how much he trusted Batman, despite their ideological differences.
- In the John Grisham novel The Partner, Patrick Lanigan stole $90 million from a corrupt law firm and their client and fled the country. Years later, knowing that he is about to be caught by mooks the client hired, he gives control of the money to his girlfriend and instructs her to inform the FBI of his whereabouts and thus ensure that he is not killed. He is aware that she could just take the money, disappear and leave him to be killed.
- In A Wizard of Earthsea Vetch tells Sparrowhawk his True Name (Estarriol) and Sparrowhawk tells Vetch his True Name (Ged) as a sign of the trust they have in each other.
- BDSM utilizes this. In fact, it's the Trope Namer. "Power Exchange" is the only constant definition in all forms of BDSM. For the submissive party, the thrill is in giving control to someone else and lacking the power to stop what they do with it, and for the dominant party, the thrill is in having control and using it for your own amusement. However, both parties must remain Safe, Sane and Consensual at all times.
- In AdventureQuest Worlds, the entrusting of the Shadowscythe Amulet to your character at the end of the Doomwood saga is very much one of these for Artix, because as the Champion of Darkness, he can be controlled with it, and you are one of the few people he trusts to use this power responsibly.
- Tedd from El Goonish Shive gave Sarah the ability to perform near-perfect identity theft of him at will. While he says it's just a token of mutual trust to his "lab rat", Tedd is a self-admitted (twice) Mad Scientist and knowingly retraces some steps of his Alternate Universe counterpart who apparently tried to kill him without any good reason, and she's the Only Sane Lass in their team, who was a victim of one irresponsible stunt he performed.
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