Cheapest priced copy (Commander 2013): $0.49 at Card Kingdom, near mint, their price list as of .
Dungeon Geists 
Creature — Spirit · 3/3 · Rare · Core Set 2020 Promos · #57p
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.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
6 printings · $0.49 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2013 | 42 | Rare | $0.49 | — |
| Commander Anthology | 39 | Rare | $0.49 | — |
| Dark Ascension | 36 | Rare | $0.49 | $0.79 |
| Core Set 2020 | 57 | Rare | $0.49 | $0.49 |
| Core Set 2020 Promos | 57p | Rare | $0.49 | $0.79 |
| Core Set 2020 Promos | 57s | Rare | — | $0.49 |
4 rulings from Wizards of the Coast
- 15 April 2013The 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.
- 12 July 2019Dungeon 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.
- 12 July 2019If 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.
- 12 July 2019If 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.