Let's not bury the lede here, Swarm is a Nazi made of bees.
Swarm is one of those characters who just seems weirder the more you know about him. Fritz von Meyer was one of Hitler's top scientists who escaped to South America following the fall of Nazi Germany. He then became a bee keeper and discovered a swarm of mutant of mutant bees.
Fritz tried and failed to enslave the queen bee and was, in return, eaten by the rest of the swarm. However, Fritz found that his consciousness had been absorbed by the mutant bees and he could now control the entire swarm. Fritz had the bees form a new body around his skeleton and set out to use his mastery over bees for evil.
This turned out to be not that impressive a power when Spider-Man is able to defeat him with a bottle of bug spray.
Swarm has continued to pop up over the years to be a mild annoyance to most heroes and an absolute nightmare for anyone with a bee allergy. He is usually beaten when heroes find a way to disrupt the mental link between him and his bees. However, in one particularly memorable encounter, he was defeated when Venom ate his skeleton, severing his bees from their focal point.