Mutant Turtles: Choujin Densetsu-hen (1996)

When the Teenage Mutant Ninja Turtles acquire Mutastones from Crys-Mu, the spirit of light, they acquire the ability to enhance themselves into Super Turtles for a duration of three minutes. Meanwhile, the evil Shredder and his minions Bebop and Rocksteady stumble upon the Dark Mutastone, which transforms them into Devil Shredder, Supermutant Bebop and Supermutant Rocksteady, respectively. But the Turtles have one more trick up their shells: all four of them can combine into their ultimate form—Turtle Saint. (Source: ANN)