Cheapest priced copy (Homelands): $23.99 at Card Kingdom, near mint, their price list as of .
Koskun Falls 
World Enchantment · Rare · Homelands · #55
At the beginning of your upkeep, sacrifice this enchantment unless you tap an untapped creature you control.
Creatures can't attack you unless their controller pays {2} for each creature they control that's attacking you.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $23.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Homelands | 55 | Rare | $23.99 | — |
3 rulings from Wizards of the Coast
- 1 February 2007In the Two-Headed Giant format, you still only have to pay once per creature.
- 1 October 2008This has the supertype world. When a world permanent enters, any world permanents that were already on the battlefield are put into their owners' graveyards. This is a state-based action called the "world rule." The new world permanent stays on the battlefield. If two world permanents enter at the same time, they're both put into their owners' graveyards.
- 1 February 2014Unless some effect explicitly says otherwise, a creature that can't attack you can still attack a planeswalker you control.