The Beyonder created an amalgamated planet where superpowered combatants could battle on two opposing sides -- one representing good, and one representing evil. This Marvel crossover was called Secret Wars.
Since the world was a patchwork of pieces from different planets, it got random what and who the heroes and villains would find during the "war".
One section held an alien village of kind people who allowed their healer, Zsaji to heal the wounds of Human Torch and later Colossus. Though this unintentionally led to Colossus breaking up with Kitty Pryde when he and the other heroes get back home.
One section held a suburb of Denver, Colorado. Three of the local citizens make their debut as future important characters. Julia Carpenter as Spider-Woman, Titania, and Volcana.
After a certain point, most of the villains lost interest in the "war" and just wanted to go home. Molecule Man used his powers to take the entire Denver suburb in a sealed, self-sustaining bubble and take the people and the villains back to Earth in near instant teleportation.
The other significant thing that happened on Battleworld is that Spider-Man would create a brand new, black costume for himself. He subconsciously designed it based on Spider-Woman's costume. some time after he and all the heroes return to Earth, Spider-Man would discover that his costume is actually an alien symbiote.
The Stranger recreated a world similar to the original Battleworld that shared the same name. Much like in Secret Wars, groups were pitted in combat against each other. After the battle, this planet was destroyed.
Secret Wars (2015)
Yet another new Battleworld was created by Dr. Doom, following the destruction of the Multiverse after a series of Incursions. It is located in Earth-15513.
In What If? (vol. 2) #114, an alternate future is explored 25 years after the events of Secret Wars. On this Battleworld, the combatants did not leave after the original crossover.