The Skullspore Nexus is a mythic Legendary Artifact card from The Lost Caverns of Ixalan (LCI), collector number 340. Current market price: $11.62 (foil $14.05), TCGplayer market prices via Scryfall.
This spell costs {X} less to cast, where X is the greatest power among creatures you control. Whenever one or more nontoken creatures you control die, create a green Fungus Dinosaur creature token with base power and toughness each equal to the total power of those creatures. {2}, {T}: Double target creature's power until end of turn.