Instill Energy
Enchantment — Aura
Enchant creature
Enchanted creature can attack as though it had haste.
: Untap enchanted creature. Activate only during your turn and only once each turn.
Dameon Willich
Rulings
Any Auras (or other effects) which are on the creature that would cause it to not be untapped (or have a cost to be untapped) during untap step do not in any way hinder or imply a cost to use this card's ability to untap once during the turn.
(wotc • 2005-08-01)
Instill Energy's untap ability will not untap the creature until it resolves. This means other spells and abilities can be used before it actually becomes untapped.
(wotc • 2004-10-04)
If attached to an opponent's creature, you can untap their creature during your turn.
(wotc • 2004-10-04)
Rulings
Any Auras (or other effects) which are on the creature that would cause it to not be untapped (or have a cost to be untapped) during untap step do not in any way hinder or imply a cost to use this card's ability to untap once during the turn.
(wotc • 2005-08-01)
Instill Energy's untap ability will not untap the creature until it resolves. This means other spells and abilities can be used before it actually becomes untapped.
(wotc • 2004-10-04)
If attached to an opponent's creature, you can untap their creature during your turn.
(wotc • 2004-10-04)