Omnath, Locus of Mana is a mythic Legendary Creature — Elemental card from Worldwake (WWK), collector number 109. Current market price: $5.73 (foil $35.62), 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.