Dead of Winter

Sorcery

All nonsnow creatures get -X/-X until end of turn, where X is the number of snow permanents you control.

2019-06-14:
The value of X is determined only as Dead of Winter begins to resolve. It won’t change later in the turn if the number of snow permanents you control changes.
2019-06-14:
Dead of Winter affects only creatures that aren’t snow creatures at the time it resolves. Creatures that enter the battlefield later in the turn won’t get -X/-X.