Icefall Regent

Text:

Flying
When Icefall Regent enters the battlefield, tap target creature an opponent controls. That creature doesn't untap during its controller's untap step for as long as you control Icefall Regent.
Spells your opponents cast that target Icefall Regent cost more to cast.

Rulings:

• 2/25/2015 Icefall Regent’s triggered ability can target any creature controlled by an opponent, including one that is already tapped. In that case, the creature stays tapped and won’t untap during its controller’s untap step.
• 2/25/2015 If another player gains control of Icefall Regent, the creature will no longer be affected by the ability stopping it from untapping, even if you later regain control of Icefall Regent.
• 2/25/2015 The ability stopping the creature from untapping will continue to apply to it even if the creature changes controllers.
• 2/25/2015 Icefall Regent’s last ability affects all spells cast by your opponents that target it, including Aura spells and spells with multiple targets. It doesn’t affect abilities.