The Beyonder is a major antagonist who debuted in season 4 of Avengers Assemble (entitled Secret Wars) - he is based upon the comic book character of the same name and likewise the entire Secret Wars saga is a modern retelling of similar comic book arcs - as the Beyonder he is one of Marvel's most powerful entities.
He was voiced by Steven Weber, and his design takes elements of both his original comic book appearance (via his white suit) and his appearance in the Spider-Man cartoon series, where he had a beard and was somewhat older in appearance than in his comic book appearances.
After the Avengers experience an incoming energy signature at New York, they wake up to find themselves in the domain of Egyptia, where they discover Avengers Tower rebuilt there. As they investigate the building, they hear a voice call them over to the conference room. They arrive there to discover the Beyonder before them, whom warmly welcomes them with a banquet, though the Avengers are unperturbed by his presence. The Beyonder explains to them that they are on Battleworld, a planet where he built from other worlds, including the Avenger's, whom he states are here for his "experiment." As the heroes questions the Beyonder about the safety of their world, the Beyonder tells them the fate of their world is unimportant as he tries to reason with them to no avail, as the Avengers close in on him and order him to change things back. The Beyonder, however, refuses and immobilizes the Avengers with his powers. He tells them not to worry, as he assures they'll thrive on Battleworld, before taking his leave.
After Loki appeared on Battleworld and decided to join the Avengers, he explains to them that he was the one who revealed Earth to the Beyonder, but then the Beyonder betrayed Loki after he also added Asgard to Battleworld to study his world as well.