The First Sliver is a mythic Legendary Creature — Sliver card from Jumpstart: Historic Horizons (J21), collector number 691.
Cascade (When you cast this spell, exile cards from the top of your library until you exile a nonland card that costs less. You may cast it without paying its mana cost. Put the exiled cards on the bottom in a random order.) Sliver spells you cast have cascade.