Gonti's Aether Heart is a mythic Legendary Artifact card from Aether Revolt (AER), collector number 152. Current market price: $1.02 (foil $5.69), TCGplayer market prices via Scryfall.
Whenever Gonti's Aether Heart or another artifact you control enters, you get {E}{E} (two energy counters). Pay eight {E}, Exile Gonti's Aether Heart: Take an extra turn after this one.