Garnon told you that destroying Tiamat's Alter Ego (the Petrification Incarnate beast at the Heart of Petrification, in this case) would grant its victor control over the heart. You tested the theory, killed the Petrification Incarnate, and returned. Garnon was amazed at your progress and sent you on to meet Adella, who told you the toughest fight was yet to come, as Tiamaranta's Eye would soon be accessible.