Magic: the Gathering - Datenbank

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 or 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.