Omnath, Locus of Mana is a mythic Legendary Creature — Elemental card from Commander Masters (CMM), collector number 310. Current market price: $4.15 (foil $5.65), 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.