Some TMNT stuff really isn't for little kids.
Chi You was a mythical Chinese emperor. In IDW's Teenage Mutant Ninja Turtles series, he is a member of the Pantheon and an enemy of the Ninja Turtles and the Ghostbusters in the crossover TMNT/Ghostbusters.
Chi-You is one of the powerful immortals who once ruled the Earth thousands of years ago. He is the brother of Kitsune and the Rat King. At some point, they mysteriously disappeared and inspired myth and legend. Ultimately, they fell from memory and man began to question if they ever really existed at all. The immortals agreed to a game to see who could regain control of Earth. Rules were drafted and only mortals could be used as their pawns. In Chinese legend and history, Chi-You was worshiped as a war god who led the Nine Li Tribe and fought Emperor Huangdi in the Battle of Zhuolu.
During era of the Mongol Empire, Chi-You approached Kublai Khan after his dominion of China had made him content. He spoke to the human warlord for three days, and planted within him a desire to invade Japan in 1274.
At some point in the Muromachi Period (1337-1573), Chi-You sensed the power of Tetsu Oni's Interspatial Teleportation Unit and journeyed across the sea to Japan to investigate if Kitsune was cheating or not. At first, he masked himself from sight but Kitsune sensed his presence and called out to him. Once Tetsu Oni stepped from the teleporter's portal, Chi-You accused Kitsune of breaking the rules of the Great Game by conspiring with a demon instead of a mortal. He declared the act forfeits her from the game and he moved to take her soul. When Tetsu Oni showed up with a canister of Mutagen, Kitsune took her chance dodged his attacks and tossed her brother into the portal. Due to safeguards in the teleportation unit on the part of the Tetsu Oni, Chi-You was not transferred to another dimension, but stranded in a state of eternal limbo.
Centuries later, Chi-You came out of his transdimensional prison when the Turtles, Casey Jones and April O'Neil accidentally entered the reality of the Ghostbusters during a test run by a teleporter constructed by Harold Lillja. He also found himself in possession of a new power, which allowed him to submit the spirit of the people directly to his will and make them his puppets. At first, Chi-You took possession of Casey, then he sought a group of strong slaves, with whom he wanted to conquer the Earth openly for himself, since he no longer saw himself tied to the subtle rules of the game of his brothers and sisters. However, after they had recognized themselves as like-minded allies, the Turtles and the Ghostbusters managed, with para-technical means, to deprive Chi-You of his new power and to imprison him in a ghost trap. Winston Zeddemore sent the trap back through a newly constructed dimensional portal into the universe of the Turtles. Chi-You sensed that he was now in the same universe as his siblings, but was still trapped.
During the Pantheon's centennial gathering, Toad Baron noted that Chi-You was still missing. A few of the Pantheon suspected Kitsune's hand in their brother's disappearance, as she was last to see him. Kitsune feigned innocence, stating that her brother accused her of cheating in their games, storming her lands and leaving as he he came. However, she omitted the information that he fell into a transdimensional portal.
- Chi You on Wikipedia