Cheapest priced copy (Game Night: Free-for-All): $0.35 at Card Kingdom, near mint, their price list as of .
Forbidding Spirit 
Creature — Spirit Cleric · 3/3 · Uncommon · Game Night: Free-for-All · #10
When this creature enters, until your next turn, creatures can't attack you or planeswalkers you control unless their controller pays {2} for each of those creatures.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $0.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Game Night: Free-for-All | 10 | Uncommon | $0.35 | — |
| Ravnica Allegiance | 9 | Uncommon | $0.35 | $0.99 |
4 rulings from Wizards of the Coast
- 25 January 2019While Forbidding Spirit’s effect is in effect, your opponents can choose not to pay to attack with a creature that attacks “if able.” If there’s no other player or planeswalker to attack, that creature simply doesn’t attack.
- 25 January 2019Because it doesn’t modify the characteristics of creatures, Forbidding Spirit’s triggered ability can affect creatures that enter the battlefield after that ability resolves.
- 25 January 2019In a Two-Headed Giant game, creatures can attack your teammate and planeswalkers your teammate controls without requiring a mana payment.
- 25 January 2019Forbidding Spirit’s effect continues until your next turn even if Forbidding Spirit leaves the battlefield before then.