Omnath, Locus of Mana is a mythic Legendary Creature — Elemental card from Commander Masters (CMM), collector number 680. Current market price: $6.84 (foil $8.52), 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.