It has three purposes. The first prevents a change in form; he cannot release his glamour. The second drains his magic and feeds it back into the collar, making him weaker and the collar stronger. And the third”—she sighed happily—“causes pain. Far more than you can even imagine. His own magic feeds the pain, along with whatever I add to it. So every time I touch him with this”—she waved her rod—“the pain will be on top of what he is already experiencing.
