Captor
From wipipedia.org
A captor in bondage or BDSM play is the person who ties up or imprisons someone else (the captive). Such a person will often, but not always, be a top, dom/domme or sadist. The captor may punish or torture the captive, but this is not always done.
In a scene, the captor may sometimes swap places with the captive; this is called a turnaround.
