Malsumis is the Abenaki god of evil, and that seems to be about all the information anyone has on the guy.
He came into being after Tabaldak created humanity (or the world, depending on the version). Either Tabaldak dusted off his hands, or the dust just fell off him after he was done, but either way the leftover creation-dust formed into Gluskab (AKA Glooscap, Gluskabe, and Glooskap) and Malsumis. Both had power to create, like Tabaldak, but since mythic twins are required to be complete opposites, Malsumis is the bad twin who uses his powers for evil, and Gluskab is the one who uses his powers for good. While Gluskab went around helping humanity out by doing things like saving the world from a thirsty frog monster and stopping an eagle who made bad weather, Malsumis went around putting thorns on plants and giving stingers to insects.
Which, in retrospect, is a bit small of him. One would think that an evil god of evil would aim a little higher than mosquito bites and rose thorns, but what do I know.