Withering Wisps

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.

Masters Edition II

At end of turn, if no creatures are in play, sacrifice Withering Wisps.
{B}: Withering Wisps deals 1 damage to each creature and each player. Play this ability no more times each turn than the number of snow Swamps you control.

Ice Age

At the end of any turn, if there are no creatures in play, bury Withering Wisps.
oo
B Withering Wisps deals 1 damage to each creature and each player. You cannot spend more o
B in this way each turn than the number of snow-covered swamps you control.

Withering 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.

If 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.

The 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.