Sidereus was a member of the Axi-Tun race that lived on the planet of Tun. He was also servant to the despot Votan, whose plan was to destroy the Charter, allowing him to have rule over the other races of the galaxy. Sidereus played a pivotal part in Votan’s plans; it was through his manipulation of computers and satellites that projected the image of the cosmic Avenger Quasar confessing to the massacre of the members of the Intergalactic Charter that were aboard the Hub. It was Sidereus’ intent to turn the other races of the galaxy against the inhabitants of the Earth.
Sidereus was in charge of the genocidal weapon known as the Cauldron of Conversion, which was used to spread deadly plagues throughout the universe. This weapon was used to kill the M'Ndavians of the planet M'Ndavi, tenth from the sun of the B'Ibwo star system. While processing the Korbinites for termination with the destructive weapon, the Axi-Tun ship was boarded by the Star Masters, who were defeated by Throk and Dampyre. Sidereus planned to throw the Star Masters into the Cauldron to further his plans of galaxy wide destruction. However the Star Masters managed to escape and Sidereus, Aldebron and the rest of the ship’s crew were instructed to commit suicide by Votan, for failing to destroy the Korbinites. They complied but Aldebron and Sidereus faked their deaths to escape, with the intention to wrest the throne from Votan in the future.
Sidereus and Aldebron have not been seen since their escape.