Lair of the Hydra is a rare Land card from Modern Horizons 3 Commander (M3C), collector number 353. Current market price: $0.36, TCGplayer market prices via Scryfall.
If you control two or more other lands, this land enters tapped. {T}: Add {G}. {X}{G}: Until end of turn, this land becomes an X/X green Hydra creature. It's still a land. X can't be 0.