Ill-Timed Explosion is a rare Sorcery card from Murders at Karlov Manor Promos (PMKM), collector number 207p. Current market price: $0.32 (foil $0.30), TCGplayer market prices via Scryfall.
Draw two cards. Then you may discard two cards. When you do, Ill-Timed Explosion deals X damage to each creature, where X is the greatest mana value among cards discarded this way.