A Sub-Worlder is a creation of the Masters of the Black Stars, a pentagon of giant planets that devour solar systems to power the giant star that glows in the middle of their flying formation.
The Sub-Worlders are a combination of organic material and some kind of metal. They are slaved created for the purpose of caring for giants sweepers that suck up the material of destroyed solar systems. They live off of the energy of their machines. They live and are able to breed, but cannot leave the machines that they work on.
After testing the metal of Thor, Avalon asks the god to help free him and his people from their masters. Thor agrees and rides one of the vacuums toward the nearest planets. They are encountered by the Protector. Avalon falls before the Protector, but is not killed. Thor dispatches the Protector quite quickly.
Avalon is powerful enough that Thor admits that he is nearly as strong as this son of Odin.
Avalon was created by Gerry Conway and John Buscema in 1974 and first appeared in Thor # 219.