Some TMNT stuff really isn't for little kids.
Takeshi was the samurai retainer of the daimyō Ashikaga Yuu, and was so fierce a warrior that Ashikaga himself even feared him. Ashikaga set a trap for Takeshi in the mountains where a legion of his armies awaited him. He slaughtered the lot of them, but at the expense of his right leg. He was saved by a mysterious mystic and had his leg restored to him; the bloodied footprint it left behind inspired the Foot Clan's name.
He formed the Foot Clan out of the remnants of the Yuu clan, and forged its power through an alliance with the witch Kitsune and the mysterious Tetsu Oni. He was aware that Kitsune intended to groom him as a vessel for the Dragon's resurrection, but had no qualms about losing himself in the process.
In the meantime, terrible atrocities were committed by the Foot clan and Oroku Maji and Masato eventually became suspicious of the clan leader's dealings with magic, especially since he never seemed to age.
After confirming that he took a strange elixir to restore his youth every month, the two men made their move. Maji attacked Tatsuo before he could take the elixir the next month, and slew him. Kitsune cursed Maji, causing Tatsuo to later be reincarnated as Maji's son Oroku Saki.
In the afterlife, Tatsuo communed with the Dragon. When Saki also appeared in the afterlife after being killed by Splinter, Tatsuo appeared before him to ensure that their common soul would become one with the Dragon. Saki continued to act independently and resist, forcing Tatsuo to capture him to finish the process. His efforts were seemingly thwarted as Saki was saved by Splinter's spirit empowered by the Dreamer. As Saki was set to be fully awakened by the Dreamer, the Dragon took Tatsuo to try and find a way to return to the world nonetheless.