His story is told in full by himself in "The Vampire Lestat", reference: VII Ancient Magic, Ancient Mysteries" pp 399 - 538, and "Marius's Story" from p 431.
Born to imortality "in the years of Augustus Caesar", at the turning point of eras, he says that he has the capability to look at things from a new angle, as is Lestat. he believes that this is why he has survived so long, where vampire law decrees he should have gone mad and killed himself, or been killed out of mercy.
Marius is of Cleltic descent, with blue eyes and fair hair. He is tall and "more finely made than other men". He was lover to Pandora and later master to Armand.
He was made a vampire to replace the God of the Grove following the exposure of Those Who Must be Kept to the sun by their then keeper, who just wanted to see what would happen. What happened was that all vampires, having originated from Akasha, were burned equally, but they being weaker than she were maimed more severely.
On escaping the Grove after the ritualistic festival conducted in his honour, Marius went to Egypt to fing the King and Queen and became their new protector. A position he did not look back on until the Queen began to "live" again.