Cheapest priced copy (Invasion): $2.49 at Card Kingdom, near mint, their price list as of .
Fight or Flight 
Enchantment · Rare · Invasion · #16
At the beginning of combat on each opponent's turn, separate all creatures that player controls into two piles. Only creatures in the pile of their choice can attack this turn.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $2.49 to $19.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Invasion | 16 | Rare | $2.49 | $19.99 |
3 rulings from Wizards of the Coast
- 1 August 2008The effect is best understood if you read it as "creatures not in the chosen pile can't attack."
- 1 August 2008Creatures which enter (or cards which become creatures) after the beginning of combat ability resolves are not placed in either pile. Since they weren't in the chosen pile, they won't be able to attack.
- 1 August 2008If you have two of these, then you will separate the creatures into piles twice, and the opponent will choose a pile each time. Only creatures that were in both of the chosen piles can attack.