Marit Lage's Slumber

Whenever Marit Lage's Slumber or another snow permanent you control enters, scry 1.
At the beginning of your upkeep, if you control ten or more snow permanents, sacrifice Marit Lage's Slumber. If you do, create Marit Lage, a legendary 20/20 black Avatar creature token with flying and indestructible.

Snow-Covered Island

Frostwalk Bastion

{T}: Add {C}.
{1}{S}: Until end of turn, Frostwalk Bastion becomes a 2/3 Construct artifact creature. It's still a land. ({S} can be paid with one mana from a snow source.)
Whenever Frostwalk Bastion deals combat damage to a creature, tap that creature and it doesn't untap during its controller's next untap step.

Rimewood Falls

({T}: Add {G} or {U}.)
Rimewood Falls enters tapped.

Hedron Crab

Landfall — Whenever a land you control enters, target player mills three cards.

Explore

You may play an additional land this turn.
Draw a card.

Glacial Revelation

Reveal the top six cards of your library. You may put any number of snow permanent cards from among them into your hand. Put the rest into your graveyard.

Snow-Covered Forest

Migratory Greathorn

Mutate {2}{G} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Whenever this creature mutates, search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

Ruin Crab

Landfall — Whenever a land you control enters, each opponent mills three cards. (To mill a card, a player puts the top card of their library into their graveyard.)

Iceberg Cancrix

Whenever another snow permanent you control enters, you may have target player mill two cards.

Growth Spiral

Draw a card. You may put a land card from your hand onto the battlefield.

Into the North

Search your library for a snow land card, put it onto the battlefield tapped, then shuffle.