Icefall Regent
Creature — Dragon
Flying
When this creature enters, tap target creature an opponent controls. That creature doesn't untap during its controller's untap step for as long as you control this creature.
Spells your opponents cast that target this creature cost 
more to cast.
David Gaillet
Rulings
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.
(wotc • 2015-02-25)
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.
(wotc • 2015-02-25)
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.
(wotc • 2015-02-25)
The ability stopping the creature from untapping will continue to apply to it even if the creature changes controllers.
(wotc • 2015-02-25)
Rulings
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.
(wotc • 2015-02-25)
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.
(wotc • 2015-02-25)
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.
(wotc • 2015-02-25)
The ability stopping the creature from untapping will continue to apply to it even if the creature changes controllers.
(wotc • 2015-02-25)