Whenever you cast a noncreature spell, you may pay. If you do, tap target creature an opponent controls and it doesn't untap during its controller's next untap step.
9/20/2014 You decide whether to pay as the triggered ability resolves.
9/20/2014 The ability can target a creature that’s already tapped. It still won’t untap during its controller’s next untap step.
9/20/2014 The ability tracks the creature, but not its controller. If the creature changes controllers before its first controller’s next untap step has come around, then it won’t untap during its new controller’s next untap step.