Some TMNT stuff really isn't for little kids.
The Dragon is first mentioned in Pantheon Family Reunion, part 1, as a character who already died in the past. Kitsune reveals that her entire involvement with Oroku Saki has been to groom him as a vessel for the Dragon's resurrection. She hopes not only to reunite her quarreling family, but to annihilate humanity in the process. Kitsune's plan immediately faces resistance: Jagwar doubts any human can be their father's vessel, and Manmoth considers their father too powerful to be harnessed. Toad Baron also does not seem excited by the prospect, noting that their father hated parties and never let his children have fun. Kitsune's goal of using their father to destroy humans cannot be shared by Aka, who loves humanity and considers them the true children of the Pantheon. Aka loved her father and loves all her siblings, but she has already severely hindered Kitsune's plans to resurrect the Dragon when she intervened to help prevent her youngest sister from stealing Oroku Saki's deceased remains from Hamato Yoshi's Foot Clan.
He makes a brief appearance when Karai meditates in the Karai's Path arc, as a ghostly dragon who warns her about the sword she is seeking. Because Karai asks him for his help with sufficient respect, he shows her the way to it.
He also appears briefly in the Inside Out story, where he appears in Leonardo's dreams and refers to himself as "the future that burns."