Myntasha, Honored One is a mythic Legendary Snow Creature — Elk card from 2019 Heroes of the Realm (PH19), collector number 5.
As an additional cost to cast this spell, you may open a sealed Magic booster pack and put the cards on the bottom of your booster pile in a random order. Spells you cast have booster cascade. (It's like cascade, but use your booster pile rather than your library.)