The Eldians are people from Earth that were kidnapped by the universe and enslaved for planetary military combat.
In a space expedition, the aliens discovered earth where humans had not yet ventured. They abducted all of them to use them as a military force in their wars with rival planets. Broccolibuttsapoklexxxxxxx is considered the most powerful planet with human combatants on its behalf. Earthlings are used because they just keep coming back – space civilians would desert or die in war much easier than those who know nothing else but fighting and being taken captive, which is why they have been called “Eldians”.