Existem vários tropos para isso:
It may shock you to know that I Have Your Index. How I came to obtain it is not important at this particular point in time. All that is important is that I'm willing to give it back . . . for a price.
Que tem dois tropos relacionados:
The Big Bad tries to get the hero (or someone else) to do his bidding by taking someone precious from him and essentially holding them to ransom. It's often a significant other, frequently a wife or girlfriend; but any family or friend works, or even strangers if it's a certain type of hero.
E o segundo, que eu achei mais relevante, é: Hostage for MacGuffin :
The villains have captured . . . someone. Anyone. Usually a family member or loved one, but it really could just be the guy down the street, because the main character loves everyone.
The Villains are willing to propose a trade. Give them the superpowered item that will let them conquer the Earth and kill far more people than just their one hostage, or they will kill their hostage.