Cheapest priced copy (Ice Age): $0.35 at Card Kingdom, near mint, their price list as of .
Withering Wisps 
Enchantment · Uncommon · Ice Age · #168
At the beginning of the end step, if no creatures are on the battlefield, sacrifice this enchantment.
{B}: This enchantment deals 1 damage to each creature and each player. Activate no more times each turn than the number of snow Swamps you control.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $0.35 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Ice Age | 168 | Uncommon | $0.35 | — |
3 rulings from Wizards of the Coast
- 1 October 2008Withering Wisps's triggered ability has an "intervening 'if' clause." That means (1) the ability won't trigger at all unless there are no creatures on the battlefield when a turn's end step begins, and (2) the ability will do nothing if there are any creatures on the battlefield by the time the ability resolves.
- 1 October 2008The number of snow Swamps you control and the number of times Withering Wisps's ability has been activated so far that turn are checked each time you attempt to activate the ability.
- 1 October 2008If all creatures leave the battlefield during a turn's End step (after any "at the beginning of the end step" triggers would have triggered that turn), Withering Wisps will remain on the battlefield because the trigger condition has already passed.