Icefall Regent — ruling 1

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.

Icefall Regent — ruling 2

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.

Icefall Regent — ruling 3

The ability stopping the creature from untapping will continue to apply to it even if the creature changes controllers.

Icefall Regent — ruling 4

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.

Icefall Regent : règles, fonctionnement et interactions (MTG)

Icefall Regent is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

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.

How it works : Icefall Regent

Original English text — Wizards of the Coast / Scryfall

#1February 25, 2015· Wizards of the Coast
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.
No other card is named in this ruling.
#2February 25, 2015· Wizards of the Coast
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.
No other card is named in this ruling.
#3February 25, 2015· Wizards of the Coast
The ability stopping the creature from untapping will continue to apply to it even if the creature changes controllers.
No other card is named in this ruling.
#4February 25, 2015· Wizards of the Coast
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.
No other card is named in this ruling.