Some TMNT stuff really isn't for little kids.
As a young boy, Saki was friends with Hamato Yoshi, but the two grew apart as they aged, both because of their differing temperaments and because of Saki's ambition.
When he was a young man, he was contacted by the witch Kitsune, who lured him to her home and revealed his past incarnation to him. Saki was enraged by the revelation of his father Oroku Maji's part in Takeshi Tatsuo's death, and murdered both Maji and Masato. He framed his crime as the doing of unknown assassins, and was made the jonin of the Foot Clan shortly afterwards.
With Kitsune's help, Saki began turning the Foot back into a bloodthirsty clan who dominated all others. He was also attacked by a time-traveling Leonardo, who was trying to save his family preemptively, but the attack by a "demon" only hardened Saki's resolve to allow no dissent among the Foot's ranks. When Yoshi refused to take part in the slaughter of innocent civilians, Saki marked him for death, and ordered for Yoshi, his wife Tang Shen and their four sons to be killed immediately.
Eleven years later, he caught up to the escaped Hamato clan. He ordered the four boys put to death before Yoshi's eyes, before beheading his former friend himself.
Some years later, Saki's quest for immortality (which dated back to his previous incarnation) was completed as Kitsune discovered a method of making him immortal. Saki defeated the Tetsu-Oni and stole a large canister of ooze from him before blowing up his portal. He was then instructed by Kitsune to drink a portion of the ooze, commit seppuku, and be submerged in a casket filled with ooze until a descendent would wake him.
While he was immersed in the ooze, his spirit traveled to a form of the afterlife in which he was told that he was destined to rule the world, but then die again, conquer the afterlife, and then be reduced to a lost soul. He also contacted his descendent Oroku Karai in her dreams, and urged her to awaken him. She did so after reviving the Foot Clan, and handed over leadership of it to her "grandfather."
He began augmenting the Foot Clan with mutated soldiers, starting with a female arctic fox, and at one point ordered two ninja to raid StockGen for their genetic experimentation samples, including four baby turtles. Some months later he was informed by an informant at StockGen of the existence of a rat mutant, Splinter, who was abducted and brought before the Foot Clan. However, Saki soon deduced that Splinter was the reincarnation of his old enemy Hamato Yoshi, and attempted to kill him. However, Splinter was saved by his Turtle sons before Shredder could do so, and Saki deduced that they were also the reincarnations of Yoshi's four sons.
He soon decided that Leonardo, being a natural leader and skilled warrior, should be his heir and second-in-command. After recovering Kitsune, Saki successfully orchestrated an elaborate plan that involved kidnapping and stabbing Casey Jones, and abducting Leonardo while his brothers rushed Casey to a hospital. When Kitsune was finished brainwashing Leonardo, Saki had a capable and ruthless new chunin.
He utilized Leonardo extensively during his war with the Savate Ninja, but was dissatisfied that Kitsune had forbidden the Turtle from killing. After capturing and executing the Savate leader Victor in front of the New York underworld, Saki was attacked by Splinter, the three other Turtles, and Old Hob. He attempted to turn Leonardo against his family again, but the brainwashing had started to deteriorate. He was then surprised by the revelation that Karai had made two new mutants from the Foot Clan ranks, Bebop and Rocksteady, and later rewarded her for her loyalty and initiative.
He also gave an ultimatum to the criminal Arnold "Hun" Jones, informing Jones that he was the one who had stabbed Casey, and that Casey had little time left unless he allied himself with the Foot.
A turning point in his conflict with the alien General Krang came when the two arranged a meeting on Krang's ship. Saki proposed that, in exchange for a supply of ooze, the Foot Clan could be the human face of Krang's domination of the world. However, Krang soon revealed that he had no use for Saki, as he planned to kill all humans on the planet, and attempted to murder the ninja lord as revenge for his past betrayal. Saki had placed a decoy in his armor, and escaped the ship with the help of his new mutants Koya and Bludgeon.
Donatello later contacted Saki with the offer of a temporary alliance against Krang, and a plan to slip a Foot Clan army onto Burnow Island. Saki agreed to this measure, but with the stipulation that Bebop and Rocksteady would be left with Donatello, to kill him if he betrayed the Foot. After Shredder discovered that Krang had been forewarned of his arrival, he ordered the two mutants to kill the Turtle. When the Technodrome activated and began terraforming the island, a wounded Saki was left to suffocate in the alien atmosphere, but was saved by the intervention of Baxter Stockman.
When Saki awoke, Stockman proposed a mutually beneficial partnership, with his M.O.U.S.E.R.s and flyborgs as evidence of his value. Saki saw this as an opportunity to gain vengeance against Splinter, and enlisted the man to invade the Turtles' lair with his creations. However, Karai destroyed the machines before they could finish their mission, much to Saki's displeasure.
Splinter then formally challenged Saki to the Gauntlet, an ancient Foot Clan ritual meant to end the feud between their clans permanently. Shredder selected Bebop, Rocksteady, Koya and Bludgeon as his representatives in the first half of the ritual; after the Turtles triumphed, he fought and defeated them all. He then fought with Splinter, who successfully defeated him in single combat, dealing Saki a mortal blow. He then committed seppuku with his claws, while Splinter acted as his second by decapitating him. After his death, Saki's body was placed in a crypt at the Foot Headquarters. His remains were eventually stolen by Kitsune; however, she found that his skull was still in Splinter's possession.
As Kitsune attempted to return life to Saki's remains, his spirit ended up wandering through different planes of the afterlife, with Splinter as his companion. He ended up in a hellish landscape where he was attacked by figures from his past lives, and encountered the demon child Kid once again. During his time there, Saki was attacked by Takeshi, who wanted him to submit to the presence of the Dragon in their shared soul. Splinter freed him from Tatsuo's grasp and brought him before the Dreamer before being reincarnated with his wife. The Dreamer told Saki the truth about himself and Kitsune's love for him, leaving him to choose his destiny. Saki chose to reject the Dragon and face him in battle, but was unable to keep the Dragon from reviving in his headless body.
Splinter then reunited Saki's head with his body, allowing Saki to reclaim his body and, through Splinter's self-sacrifice, his own life. This had the effect of killing the Dragon, while reuniting a newly-repentant Oroku Saki with Kitsune.