Undercity Plague is a rare Sorcery card from Gatecrash (GTC), collector number 83. Current market price: $0.19 (foil $0.61), TCGplayer market prices via Scryfall.
Target player loses 1 life, discards a card, then sacrifices a permanent of their choice. Cipher (Then you may exile this spell card encoded on a creature you control. Whenever that creature deals combat damage to a player, its controller may cast a copy of the encoded card without paying its mana cost.)