At the beginning of the end step, if no creatures are on the battlefield, sacrifice Withering Wisps.
: Withering Wisps deals 1 damage to each creature and each player. Activate this ability no more times each turn than the number of snow Swamps you control.
10/1/2008 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.
10/1/2008 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.
10/1/2008 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.