Cheapest priced copy (Duel Decks: Merfolk vs. Goblins): $0.49 at Card Kingdom, near mint, their price list as of .
Tidebinder Mage 
Creature — Merfolk Wizard · 2/2 · Rare · Magic 2014 · #73
When this creature enters, tap target red or green 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
2 printings · $0.49 to $2.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duel Decks: Merfolk vs. Goblins | 22 | Rare | $0.49 | — |
| Magic 2014 | 73 | Rare | $0.49 | $2.49 |
4 rulings from Wizards of the Coast
- 1 July 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.
- 1 July 2013If 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 Tidebinder Mage.
- 1 July 2013If you lose control of Tidebinder Mage between when it entered the battlefield and when the ability resolves, the target creature will become tapped. However, it won’t be affected by the other effect and will untap as normal during its controller’s untap step.
- 1 July 2013If another player gains control of Tidebinder Mage, the creature will no longer be affected by the ability preventing it from untapping, even if you later regain control of Tidebinder Mage.