captor
A person who captures someone and keeps them prisoner.
A captor is someone who captures and holds another person against their will. When kidnappers take someone hostage, they become that person's captors. When soldiers take prisoners during wartime, they're acting as captors. The word describes anyone who keeps another person confined or controlled.
The key element is the loss of freedom: the person being held, called the captive, cannot leave when they want to. A captor might lock someone in a room, tie them up, or simply prevent them from escaping. In adventure stories, heroes often have to outwit their captors to escape from dungeons or enemy camps.
You'll also see this word in historical contexts. During wars, captured soldiers become prisoners of war, and their captors are responsible for treating them according to international rules. In stories about historical conflicts, you might read about captives trying to escape from their captors or negotiate their release.
While a captor is the person doing the holding, a captive is the person being held. Think of it as a pair: every captor has at least one captive, and every captive has at least one captor.