The Underminer is a supervillain appearing at the very end of The Incredibles, prompting the Parr family to spring into action and serving to demonstrate that they will go on to have many adventures and fight various enemies as a superhero team. His ambition is to destroy peace and joy. He appears with greater roles in the spinoff video game The Incredibles: Rise of the Underminer and the later Incredibles comics by BOOM! Studios.
Role in the film
The Underminer's past is thought to be that he was once a mole, who after mutation, decided to get revenge on the "surface dwellers" from above. All he ever had to base off his supervillain-ness is old movies.
The Underminer was presumably killed when Mr. Incredible and Frozone throw his robot creation in which he was on top of into his lair which exploded and destroyed his robot army ending his plans.
In the comic series, The Underminer makes an appearance in the "Terror from Beneath" story arc, in which he briefly allies with Dash to fight off alien invaders. However, the appearance is revealed to be part of an extensive hypnosis-induced dream. He later makes an actual appearance as a member of the Unforgivables, the criminal alliance headed by Xerek. As part of a plan to ruin the reputation of the Incredibles, Underminer is given control of a giant robot painted in the Incredibles' colors and rampage through Metroville. Xerek later eliminates Underminer upon completing his purpose.