The following contains major spoilers for Teenage Mutant Ninja Turtles #131, available now from IDW.
While the past few months have seen the Teenage Mutant Ninja Turtles take on a myriad of new threats, nothing they have encountered so far could possibly prepare them for the battles to come. The heroes in a half shell will not only have to face their own inner demons if they want to stand any chance of survival, but their steadfast leader will have to go much further than that. It appears that Leonardo overcoming his worst self won't be as simple as striking it down. Instead, he's going to have to embrace everything the Foot Clan ever taught him, and there is no way to tell which Leo will emerge once he does.
As the threat of the Rat King's Armageddon Game looms overhead, the eponymous heroes of Teenage Mutant Ninja Turtles #131 (by Kevin Eastman, Tom Waltz, Sophie Campbell, Pablo Tunica, Ronda Pattison, and Shawn Lee) have embarked upon a dangerous new kind of training. Under the guidance of Oroku Saki, the former Shredder and leader of the Foot Clan, the Turtles have all come face-to-face with the grotesque horrors that haunt them. At least, all but Leonardo have battled their inner demons, yet his own impending confrontation is poised to be unlike anything his brothers have seen.
Rather than fighting any physical or mystic battles, Leonardo is poised to go to war with the worst version of himself. As Oroku Saki explains, Leo will have to dig deep to uncover the mutant he once was after dedicating himself to the Foot Clan. From there, it won't be a matter of killing that version of himself that determines the course of the rest of his training. In contrast to the other Turtles' experiences, Leo's challenge will be one of coming to terms with what he did under the Shredder's leadership, and there is no guarantee which version of him will ultimately emerge after the fact.
Shortly after the Turtles first journey into General Krang's war-torn Dimension X, Leonardo was capture by the Foot Clan and subjected to the machinations of the ancient witch Kitsune. After being convinced that his brothers were dead, and his father had abandoned him, Leo embraced his place as the chunin of the Foot. His propensity for lethal violence in this state made him instrumental in bringing down the then budding Savate ninja clan, although it also drove a major wedge between himself and the rest of his family. Even after Leo was freed from Kitsune's control, returning to his old life was more difficult than he ever let on.
Now it seems as though Leonardo never really let go of his life under the Foot. Although there isn't any clear indication of what that means for his future. When the other members of the Splinter Clan overcame their own demons, it was apparent that doing so was a milestone in their mystical training. Oroku Saki had warned them of the dark spirits they would likely be encountering, and thanks to his help they understood what they needed to do. Assuming the former Shredder is being just as honest with Leonardo, what comes next could change his life forever.
It's hard to imagine that allowing his darker self to emerge, let alone giving in to that persona, won't have a lasting impact on Leonardo. Even if he manages to overcome his past and lay to rest all the guilt and hatred he has tried to bury, Leo might not be all that much better for it. In fact, there is a good chance that the experience will leave him in a worse place than he has been for years, especially if embracing his old ways will help to put a stop to the Armageddon Game. Luckily, Leonardo still has the rest of the Splinter Clan at his side, and with any luck they'll be able to pull him back from the brink before it's too late.