Omnath, Locus of Mana 
Legendary Creature — Elemental
You don't lose unspent green mana as steps and phases end. Omnath gets +1/+1 for each unspent green mana you have.
Legality
standardNOT LEGALpioneerNOT LEGALmodernLEGALlegacyLEGALvintageLEGALcommanderLEGAL