There are already a deluge of good mana-doubler effects in Green, like Zendikar Resurgent, Regal Behemoth, and Nissa Who Shakes the World, with other ways to achieve similar effects by dipping into artifacts and secondary colors
My problems with it are that it has a high mana value, so that upkeep trigger wont always be all that impactful, and the activated ability also hits your own creatures
This pressures planeswalkers and players alike and is great for midrange decks
Hands down the best sac outlet in the game, Viscera SeerViscera Seer would be excellent if it was just a one-mana creature that said "sacrifice a creature" on it, as evidenced by Carrion FeederCarrion Feeder
Re-casting the Ring from your hand lets you keep the protection from everything active, shielding you from targeted combo kills for another full round