Cover of Winter's replacement effect prevents combat damage creatures would deal to you and to attacking or blocking creatures you control.

If multiple creatures would deal combat damage to you and your creatures at the same time, this prevents X of the damage that would be dealt by each of them. This prevention is applied creature by creature. For example, if Cover of Winter has two age counters and you're being attacked by an unblocked 1/1 and an unblocked 4/4, this will prevent 2 of the combat damage that would be dealt by each one. All damage from the 1/1 is prevented and 2 damage from the 4/4 is prevented, meaning you're dealt 2 damage.

If a creature has double strike, Cover of Winter prevents X of the combat damage it would deal during each combat damage step.

If a creature's combat damage is assigned to multiple recipients (for example, if it has trample and is blocked by two creatures, and damage is assigned to each of those creatures and to you), you decide how to divide the prevention effect. You may prevent damage that would be dealt to the first creature, the second creature, and/or you, as long as the total amount of that creature's damage that would be prevented is X.