Some TMNT stuff really isn't for little kids.
Wyrm is an aggregate mutant organism that came about when many flatworms were exposed to mutagen. Rather than individually evolving, they became a single-minded creature with many small bodies combined.
Wyrm developed in the sewers over many months, during which it witnessed the Turtles going about their daily lives. It also amassed a large collection of lost "pretty things" that it collected in a massive warehouse-like room in the sewers. These objects seemed to be used as a substitute for actual friends and family, since Wyrm declared that it would only be "alone" if it didn't have its things.
Wyrm only became known to the Turtles when Michelangelo, coming back home with a pizza, stumbled across Officer Maureen Lin searching through the sewers for her missing brother-in-law (who had been murdered by Wyrm). After abducting Lin, Wyrm severely beat Michelangelo while taunting him about how it was the "rot in the shell" that had infected his sewers.
Michelangelo made his way to Wyrm's lair and found Lin there. A fight broke out between the worm aggregate and the mutant turtle. NYPD Officer Lin ended the battle by shooting a tank of gas, causing an explosion and a fire to break out. Wyrm died trying to protect its possessions from the fire, but a lone flatworm was shown to have survived.
It had a strong antipathy towards humans for "squishing" the worms, and seemed to have contempt for Michelangelo as well. This may have been partly because of its inability to understand true interpersonal relationships, like those Michelangelo has.