At the beginning of the end step, if no creatures are on the battlefield, sacrifice this enchantment.

: 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.
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.