Masked Mutant

The Masked Mutant is a sadistic supervillain who appeared in the twenty-fifth Goosebumps book, Attack of the Mutant.

Powers
The Masked Mutant was a comic book villain with the power to alter his molecular form to transform into any solid. There was a set-back, though. Once he transformed into any kind of liquid, he could not ever change back to his normal self. The villain was always trying to annihilate the League of Good Guys, his arch-enemies. His costume is sort of similar to Batman's, but with some differences: the Mutant's cowl doesn't have bat ears, there are no sleeves on the suit, the color scheme is dark blue and orange (dark purple and yellow in the video game), and in the game there are yellow lenses over the eye holes of the cowl.

Attack of the Mutant
In the book, a comic book collector, Skipper Matthews is a fan of Masked Mutant comics. Skipper once infiltrated his building and found the Galloping Gazelle, the mutant's Nemesis. He freed the superhero, but when they entered the Mutant's office, the cowardly hero retreated. The Masked Mutant prepared to kill Skipper, but Libby, a girl Skipper met, entered the office, with a toy gun aimed at the Mutant. She revealed that it was a Molecule Melter, and melted the Masked Mutant's molecules, killing him.

However, Libby revealed that there was no Libby, and she was the real Masked Mutant, and the one who was melted was one of his henchmen. Skipper tricked the Mutant into believing he was Elastic Boy and the only way he could be destroyed was by boiling sulphuric acid. The Masked Mutant transformed into a wave of sulphuric acid, but Skipper avoided the Mutant. The Masked Mutant could not revert to his original form and was defeated once and for all.

Video Game
In the video game version, the Masked Mutant is bent on transforming the real world into a comic book. He has constructed a "Green Beam" machine that can convert people and buildings into comic book drawings. When the villainous Mutant encounters the player during the final confrontation, the player destroys a tank of "Slima-Bean Juice" behind him. He is drenched in the green ooze and starts sobbing, as he shrinks away into nothing. Everything affected by the Green Beam reverts back to normal afterwards.