Sunfall is a rare Sorcery card from March of the Machine Promos (PMOM), collector number 40p. Current market price: $0.73 (foil $0.83), TCGplayer market prices via Scryfall.
Exile all creatures. Incubate X, where X is the number of creatures exiled this way. (Create an Incubator token with X +1/+1 counters on it and "{2}: Transform this token." It transforms into a 0/0 Phyrexian artifact creature.)