(Important: this article deals with the Beyonder as the character existed prior to later retcons, for other uses of Beyonder please visit his disambig page)
Beyonder was an incredibly powerful alien entity from Marvel comics who would becoming a recurring antagonist for several story-arcs, he was considered to be the single most dangerous threat the collective Marvel multiverse had ever faced and was compared numerous times to a Supreme Being whose power so greatly eclipsed those found in Marvel that even cosmic entities such as the Living Tribunal were considered inferior to him.
Due to the limitless nature of his power this version of the Beyonder would eventually be retconned into a more limited (yet still unimaginably powerful) character with varied origins over the years, varying from an immature Cosmic Cube to a Deviant.
In his original incarnation the Beyonder was stated to be the only life-form of his own personal multiverse, known as the Beyond, growing bored of his immortal existence alone he would cross over into the main Marvel multiverse and began to explore the cosmos, yet due to his omnipotent powers he quickly became a multiversal threat who endangered countless heroes and villains, even going as far as once (infamously) "killing" the cosmic entity, Death, though quickly returning "her" to "life" after realizing the imbalance he had caused.
Beyonder, due to his unchecked power, was both a villain, a hero and more often a neutral entity in his numerous appearances, the power of the Beyonder also helped to power villains such as the Molecule Man and God-Emperor Doom.