Rimewind Cryomancer

{1}, {T}: Counter target activated ability. Activate only if you control four or more snow permanents. (Mana abilities can't be targeted.)

Arcane Signet

{T}: Add one mana of any color in your commander's color identity.

Snow-Covered Swamp

Rogue's Passage

{T}: Add {C}.
{4}, {T}: Target creature can't be blocked this turn.

Doubling Cube

{3}, {T}: Double the amount of each type of unspent mana you have.

Blizzard Specter

Flying
Whenever Blizzard Specter deals combat damage to a player, choose one —
• That player returns a permanent they control to its owner's hand.
• That player discards a card.

Upwelling

Players don't lose unspent mana as steps and phases end.