Marit Lage's Slumber is a rare Legendary Snow Enchantment card from Modern Horizons (MH1), collector number 56. Current market price: $0.55 (foil $2.20), TCGplayer market prices via Scryfall.
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.