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.
1/22/2011 If another player gains control of Dungeon Geists, the creature will no longer be affected by the ability preventing it from untapping, even if you later regain control of 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.