Some TMNT stuff really isn't for little kids.
The Dragon and the Dreamer were created from Brahma as dual aspects of his personality: the Dreamer to maintain and the Dragon to destroy. The siblings grew isolated from their father's omniscience and became creators themselves to understand him better. The Dragon, however, was never satisfied and his sister helped him design a Pantheon of demigods wise and powerful enough to manipulate and intervene in the settlement of creation to ease his non-conformity.
The Dreamer went on to revert to an infant form inside a tree within the afterlife, dreaming and living through the lives of humans. The Dragon could not accept this and infiltrated her dreams, influencing mankind to perform the most terrible deeds of their history, her nightmares. In order to stop him, the Dreamer designed a mortal soul as a prison for him, binding him to the same cycle of death and rebirth that all living beings suffer: the soul of Takeshi Tatsuo, later reincarnated as Oroku Saki.
The Dragon briefly appears in in Leonardo's dreams, where he refers to himself as "the future that burns".
During their centennial Pantheon family reunion, Kitsune reveals to her siblings 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 Splinter's Foot Clan.
The Dragon later makes a brief appearance when Karai upon her quest to seek the Kira no Ken. The ghostly dragon warns her about the sword, but shows her the way to it when she asks him for his help with sufficient respect.
Upon Saki's death at the hands of his childhood friend and later enemy Splinter, the Dragon communed with Tatsuo fully possess his soul and be free of his imprisonment. Saki, however, continued to act separately from his original incarnation and defy his intentions. Tatsuo managed to defeat Saki and bring him before the Dragon to complete the process, but was thwarted by Splinter, empowered by the Dreamer. Having lost Saki, the Dragon tried to take Tatsuo nonetheless to find a way to return to the world. He confronted Saki in battle and his sister until Saki seemingly gave in.
In the world, Kitsune killed Aka to lay the path for the Dragon's return. She later used the Kira no Ken on Saki's headless body to finally act as his vessel. With Saki's body now that of the Dragon Lord, she returned the Kira no Ken to the gods and began fully summoning her father to Earth. However, Kitsune's plans to sacrifice Karai to untether him from the netherworld were thwarted and Splinter resurrected Saki at the cost of his own life. With Saki now free from his influence, the Dragon lost his vessel and was defeated, his body left dead over New York.