Cover of Winter

Cumulative upkeep [Snow] (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it. [Snow] can be paid with one mana from a snow permanent.)

If a creature would deal combat damage to you and/or one/more creatures you control, prevent X of that damage, where X is the number of age counters on Cover of Winter.

[Snow]: Put an age counter on Cover of Winter.