Valgavoth's Onslaught is a rare Sorcery card from Duskmourn: House of Horror Promos (PDSK), collector number 204p. Current market price: $0.33 (foil $1.06), TCGplayer market prices via Scryfall.
Manifest dread X times, then put X +1/+1 counters on each of those creatures. (To manifest dread, look at the top two cards of your library, then put one onto the battlefield face down as a 2/2 creature and the other into your graveyard. Turn it face up any time for its mana cost if it's a creature card.)