2013-04-15 : 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.

2019-07-12 : Dungeon Geists’s triggered ability can target a tapped creature. If the targeted creature is already tapped when it resolves, that creature just remains tapped and won’t untap during its controller’s untap step.

2019-07-12 : If Dungeon Geists leaves the battlefield before its triggered ability has resolved, the target creature will be tapped, but it will be able to untap as normal.

2019-07-12 : If another player gains control of Dungeon Geists, its effect expires. It won’t keep the creature from untapping anymore, even if you later regain control of Dungeon Geists.