Thelius was one of the Uranian branch of the Eternals who physically resembled Robert Grayson AKA Marvel Boy, formerly a guest of the Eternals. Unknown to Grayson and his father, the headband which the Eternals had given to Grayson was actually transmitting his memories into the mind of Thelius, whom the Eternals intended to use as a backup Marvel Boy if Grayson, their 'ambassador' to Earth, became lost or was killed. Following the destruction of the Uranian Eternals, Thelius went mad and, apparently believing himself to be Grayson, went to Earth as The Crusader in order to seek revenge on the humans he blamed for not saving his people. He fought the Fantastic Four, but was destroyed when his Quantum Bands overloaded.
A construct duplicate of Crusader was later created by the omnipotent Thanos using the Infinity Gauntlet, using Eon's corpse as fuel for his creation. Calling himself the Blue Marvel, this insane being sought to destroy Quasar and "reclaim" the Quantum Bands.