Omnath, Locus of Mana is a mythic Legendary Creature — Elemental card from From the Vault: Legends (V11), collector number 7. Current market price: — (foil $9.17), TCGplayer market prices via Scryfall.
You don't lose unspent green mana as steps and phases end. Omnath gets +1/+1 for each unspent green mana you have.