My Forces Are Innumerable is a common Ongoing Scheme card from Archenemy: Nicol Bolas Schemes (OE01), collector number 11★. Current market price: $8.19, TCGplayer market prices via Scryfall.
(An ongoing scheme remains face up until it's abandoned.) At the beginning of your end step, create a 3/3 black Horror creature token. At the beginning of your opponents' end step, they may sacrifice two creatures of their choice. If they do, abandon this scheme.