Dungeon Geists | Magic: The Gathering
Formats Dungeon Geists is Legal in
Official Oracle Text for Dungeon Geists
When Dungeon Geists 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 Dungeon Geists.
Official Rulings for Dungeon Geists
1/22/2011 : If you gain control of the creature that's being prevented from untapping, that creature won't untap during your untap step for as long as you control Dungeon Geists.
4/15/2013 : The ability can target a tapped creature. If the targeted creature is already tapped when it resolves, that creature just remains tapped and doesn't untap during its controller's next untap step.