Onslaught is an extremely powerful super-villain from Marvel comics who started primarily as an antagonist of the X-Men but would become a company-wide threat who temporarily killed a large chunk of Marvel's superhero population before said heroes would be revived later in "Heroes Reborn".
Onslaught itself would be reborn several times over the years but never again reached the same godlike level it obtained post "Heroes Reborn".
History[]
Onslaught was a malevolent thought-form born of an act of severe transgression by Charles Xavier, who used his psychic powers to put Magneto in a coma, this caused a volatile merging of Xavier and Magneto to manifest and gain its own sapience, growing more radical than its two "fathers" the creature that named itself Onslaught decided that both humans and mutants would need to be exterminated.
Onslaught thus went on a terrible rampage, gathering many powerful agents to work with it out of fear or opportunity, in addition Onslaught captured and absorbed the powers of the god-mutant known as Franklin Richards, massively increasing its already formidable powers and making it a true god that began to destroy the world.
While Franklin would be separated and Onslaught's armour shattered by a fully-enraged Hulk the entity would grow into an astral horror that once again threatened to destroy the world, worse still was any mutant who attacked would simply of made him grow in power, this forced all non-mutant heroes to rush in and attack the monster, finally weakening it enough for the X-Men to finish the job but at a terrible cost as many of Earth's heroes seemingly perished in the process.