Some TMNT stuff really isn't for little kids.
The events of Vengeance, part 6 saw the conclusion of the Gauntlet pitting the Foot Clan and its jōnin Oroku Saki against Clan Hamato led by Saki's Foot Clan brother Hamato Yoshi, who had since been reincarnated in the body of Splinter. With Saki defeated, Splinter ritually killed and beheaded Saki, then accepted Foot Chūnin Oroku Karai's offer to surrender clan leadership to him.
In truth, Yoshi had always loved Saki like a brother, and regretted their fallout and the centuries of vendetta between them. So, in honoring Saki both as the Clan's previous jōnin and as his clan brother, Splinter interred Saki's mortal remains in a tomb at Foot Headquarters.
Jennika, a Foot Assassin who had been a devout follower of Master Shredder during his leadership of the Foot Clan, had recently conspired with Darius Dun and the Street Phantoms to mount a coup against Splinter's new leadership. She was defeated, but Splinter's words helped her realize her error, and she accepted a demotion from the Assassin Caste to ordinary Foot Ninja as punishment of her treachery. In the story arc Fox Hunt, Splinter tasked Jennika with the task of guarding Oroku Saki's tomb, reassuring her that this was no menial task.
This proved true as Kitsune attempted her own coup within the Foot Clan, hoping to seize Saki's remains and resurrect her Dragon Warrior. Kitsune, using Alopex as her psychically-controlled puppet, did manage to subdue Jennika and seize Saki's burial box, then escaped from the roof in a helicopter. But when Kitsune opened the box, she found not Saki's skull, but the Shredder's iconic helmet, as Splinter had secretly switched out the skull in full anticipation that Kitsune would try and succeed in stealing it, also proving himself a shrewd mortal player in the Pantheon's ancient game. Kitsune did manage to steal the rest of Saki's body.
Jennika, for her part, had failed to protect the tomb's burial box, but fought bravely in defense of her Master Splinter even as many others succumbed to Kitsune's psychic interference, achieving further redemption in his eyes and proving herself a friend and ally of Splinter's sons the Turtles.