Some TMNT stuff was always meant for grownups.
The Utroms are a species of small, tentacled aliens, vaguely resembling brains. They were stranded on Earth and interacted with humans through a front company, T.C.R.I. (Techno-Cosmic Research Institute). They are responsible for creating and losing the canister of mutagen that transformed Splinter and the Turtles. Eventually, they were forced to leave Earth by teleporting themselves to another world.
The Turtles met the Utroms the first time shortly after their first clash with April O'Neil, Baxter Stockman and his Mouser robots. Splinter had been attacked by the Mousers and was badly bruised, he was found by aliens tucked into human-looking robot exoskeletons (TMNT Vol.1 # 3). In their search for Splinter, the Turtles stumbled across the TCRI building, and as the initials of the company were the same as those on the canister of the mutagen, which had turned them into mutants, in agreement, they decided to investigate the building in more detail. They also met the aliens, which developed into a massive brawl that ended with the Turtles being catapulted to another galaxy by a strange machine where they had some clashes with the Fugitoid, the Triceratons and the Army of the Human Federation, before the Utroms could get them back again with their teleporter (TMNT Vol.1 # 4-6).
It turned out that the Utroms are peace-loving in nature and had brought the injured Splinter into their base to help him. They also revealed the secret of the mutagen: Twenty years ago, the Utroms had crashed on Earth. Using the technology, they were able to save from the wreck, mainly their exoskeletons to move freely among men, they eventually founded the company TCRI as a base. The canister with the mutagen that transformed the Turtles, was lost in an accident, as 15 years and 1/2 years ago the Utroms were moving the last salvage from their ship into the city by truck when a blind man strode into the path of the truck. When dodging, the man a canister was jarred loose from the back of the truck. When the stranded Utroms were finally able to communicate with their homeworld, they were ordered by their leader to stay and continue observation of Earth.
But activating the teleporter had alerted the police and the National Guard, and the TCRI building was finally stormed. The Utroms immediately initiated a forced evacuation and escaped just by using the teleporter, and the Turtles and Splinter escaped - right into April's bath - before the building was destroyed by a self-destruct mechanism (TMNT Vol.1 # 7). Later, however the Utroms returned to establish a base on Earth and open official relations with humanity and with other alien cultures (in Volume 4 of the comic).
However, not all Utroms one and all peaceful. Initially portrayed as a uniformly peace-loving race, there are also some splinter groups that are considered criminal with radical ideas, recent comic books have shown several elements of their culture, some unsavory.
- Utrom Preservi: Described as a secret and illegal society of Utrom memory preservers, they steal the bodies of dead Utroms and extract their memories for archival. They appear in Tales of the TMNT vol. 2.
- Utromi Obscura Secreti: A black-ops group on the payroll of the Utrom government, carrying out jobs it does not want made public. Little is known about the group, although it has been confirmed that they were responsible for covering up the existence of the original Utrom homeworld, Magonia. It is also speculated by other races that they secretly work to prevent other races from developing teleportation technology.
- The Illuminated: A group of rogue and fanatical Utroms who believe that peace through the galaxy can only be achieved through the total annihilation of violent species. To that purpose, they kidnapped Leatherhead with the intention of using him as a template for a clone army. They were stopped thanks to the efforts of the turtles and Emissary Rikby.
It has also been revealed that the Utrom were responsible for the extinction of the J'Gel, a race of shape-shifting aliens, which, thanks to the Utroms, had spread through the galaxy, consuming entire planets.