White Sun's Twilight is a rare Sorcery card from Phyrexia: All Will Be One Promos (PONE), collector number 38p. Current market price: $4.30 (foil $1.65), TCGplayer market prices via Scryfall.
You gain X life. Create X 1/1 colorless Phyrexian Mite artifact creature tokens with toxic 1 and "This token can't block." If X is 5 or more, destroy all other creatures. (Players dealt combat damage by a creature with toxic 1 also get a poison counter.)