Primordius has so far only appeared on Earth 20051, the Marvel Adventures universe.
Primordius, a Herald of Galactus from "eons ago," says that he "failed to please Galactus...and he sent me away in disgrace!" The Fallen One has not been seen in the Marvel Adventures universe, but if the Fallen One was the first Herald of Galactus in the 20051 universe, as he was in the main 616 universe, then Primordius must have been a Herald between the Fallen One and the Silver Surfer.
In Primordius' later travels, he constantly heard about the much-heralded Silver Surfer, whose feats are respected by all. Primordius lived in constant jealously, until finally one day he attacked.
He sent a machine to Earth that was intended to absorb the Surfer's powers, so that he could more easily defeat him. However, Spider-Man happened to swing by at that exact moment, and instead the two switched powers and temporarily lost their memories. Spider-Man ended up flying out to Primordius on the Surfer's board and fought him using the Power Cosmic. They eventually returned to Earth while still fighting. The Surfer was able to merge himself with Spider-Man, and the new combined being defeated Primordius using cosmic webbing. Primordius begs them to kill him, but instead they throw him back out into space, admonishing him to spend more time living life in the moment, and less worrying about other people.
We last see him floating in space, still wrapped up in the cosmic webbing.
Primordius appears somewhat like a giant squid with six tentacles and a skull-like head, which is surrounded by a penumbra of fire. His tentacles, thorax, and head are covered by a kind of technological armor. His two front tentacles have fire constantly coming out of a hole near the tip.
Powers and Abilities
Primordius presumably has some degree of the Power Cosmic, bestowed by Galactus upon all his Heralds. He can fly through space unaided, and is able to travel great distances, having crossed "ten thousand galaxies" looking for the Surfer.
He is long lived, having served as a Herald "eons ago."
He can vent a fiery energy blast from his mouth and two front tentacles.
He used a complex device capable of stealing the Silver Surfer's powers. It is unknown whether he created this device himself, but if so, that would imply a high degree of technological knowledge and skill.