Souls of the Lost is a rare Creature — Spirit card from The Lost Caverns of Ixalan (LCI), collector number 369. Current market price: $0.39 (foil $0.46), TCGplayer market prices via Scryfall.
As an additional cost to cast this spell, discard a card or sacrifice a permanent. Fathomless descent — Souls of the Lost's power is equal to the number of permanent cards in your graveyard and its toughness is equal to that number plus 1.