Some TMNT stuff was always meant for grownups.
In an alternate reality, a man named Lucks acquired the right to be able to lead the world in return for the protection he could offer humanity during an apparent invasion of extraterrestrials. After the governments of the Earth had agreed, Lucks called upon a team of scientists lead by his friend the brilliant but conniving Dr. Shreddarius to develop a collection of bio-mechanical soldiers the Terrorkinetics. These robotic protectors were commissioned by the corrupt Chairman Lucks to defend the Earth from supposed alien Invaders. In reality, they were constructs of evil design.
Lucks, a World Unifying Leader, had a definitive hold on this universes' power and was untrustworthy. Doctor Shreddarius' own private experiments included the targeted mutation of four turtle Super Soldiers in case Lucks decided to betray his scientific friend and his creations: Griddex, Blobboid, Graviturtle, and Shellectro, who would eventually band together as The Super Turtles. The chemical mixture he used mutated the turtles, however, the solutions eventually became unstable, and an explosion rendered Shreddarius severely disabled. The four turtles received not only a humanoid shape and intelligence but also super powers; and a somewhat familiar lab rat licking the wounds and absorbing the chemicals on Shreddarius would mutate into the militant master, henceforth called Sliver.
The rat, began to teach the Super Turtles the use of their powers, but unlike the warm, caring, nurturing, disciplined and kind wisdom and love conveyed to the Ninja Turtles by Master Splinter, Master Sliver is cold and relentless in his teachings. A harsh taskmaster, his treacherous methods in training Shreddarius' Super Turtles almost destroyed them numerous times in the insane effort to gauge their limitations against the Terrorkinetics.
Finally, two scientists, Jonas Case and Neil O'Malley, would convince the Super Turtles of the truth behind Luck's evil, twisted hold on power, Shreddarius' subservience to maintain it, and that the invasion was just one of Luck's facades to adapt to the world. Sadly, Master Sliver was so blindly obedient to the mad doctor, that he would not waiver that loyalty. Defeated briefly by his former students in time, yet independent from his Master Shreddarius, The Sliver would return to his master's side. The Super Turtles then opposed their creators and made them harmless; Shreddarius, however, managed to escape, and from then on he and the Terrorkinetics became fierce enemies of the Super Turtles.
In the Mirage comics, Sliver seems to have no special abilities, apart from his human intelligence and his skill in fighting.