Withering Wisps | Magic: The Gathering
Formats Withering Wisps is Legal in
Official Oracle Text for 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.
Official Rulings for Withering Wisps
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.