Winter Is Coming

When you cast Winter Is Coming, each player exiles
a creature they control.
At the beginning of each players upkeep, that player chooses target land they control. That lands becomes
a snow land in addition to it's other types and has
"{t}:Add {s} to your mana pool." If a player controls
4 or more snow lands, that player also sacrifices a creature.
"Winter is coming."
—Lord Eddard Stark