The Hulk x Doomsday + Story

Fallout's origins by MoonKnightFan101:
Long ago, on the desolate planet Krypton, a scientist named Ban-Er was caught in a nuclear meltdown, but instead of getting obliterated, he was transformed into a green monster with bone protrusions coming from his body. It turned out an entity from the depths of hell itself called Doomsday dwelled and inhabited the gamma radiation, so when Ban-Er got exposed to it, the door was open for the Doomsday entity to inhabit a host and now, whenever professor Ban-Er got angry, he would turn into the monster and cause havoc. At one point he would end up on Earth and Earth's heroes would finally beat them, at the cost of multiple  casualties to the monsters rage. Luckily they would help the professor control the monster within him, and he would even get recruited as one of them... eventually.

Both characters belong to Marvel and DC respectively.