Victim
From wipipedia.org

In a BDSM or bondage scene, the victim is the person who is captured, tied up or suffers the punishment. A victim is often, but not always, a sub.
If the emphasis is on the bondage or imprisonment, it may be better to use the term captive.
It is possible for both or all the players in a scene to become victims. For example, they may take turns to punish or tie up each other, or there may be a turnaround.