Oracle Text

Whenever a creature an opponent controls becomes the target of a spell or ability you control, gain control of that creature for as long as you control Willbreaker.

Card Rulings

6/22/2015 If Willbreaker leaves the battlefield, you no longer control it, and its control-change effect ends.
6/22/2015 If you lose control of Willbreaker before its ability resolves, you won’t gain control of the creature at all.
6/22/2015 If another player gains control of Willbreaker, its control-change effect ends. Regaining control of Willbreaker won’t cause you to regain control of the creature.