Magnus was a vampire who made Lestat de Lioncourt his sole vampire fledgling as told in The Vampire Lestat.
Magnus is characterized by his black hair and eyes of the same color. As a mortal, he was an alchemist. In his old age, stooped and deformed by the natural wasting of his bones, Magnus had been denied the Blood by the vain Rhoshamandes, who at that time quietly ruled the Undead of France, never allowing their numbers to become unmanageable. Bitter, angry and not to be outdone, Magnus had managed to steal the Blood from a young acolyte of Rhoshamandes known as Benedict. He dug up the slumbering vampire, bounding him in heavy chains in a catacomb at the crucial time right before dusk. As the sun set, Magnus drank the blood from the helpless vampire, stealing the Dark Gift for himself, making him into an immortal vampire.
Over time, he was driven mad from his vampiric nature and immortality. For years, Magnus sought an heir to inherit the immense wealth he had accumulated over the years before ending his own life. In his dungeon he had hundreds of decomposing bodies, every one of these candidates shared the same blond hair and blue eyes as Lestat's. After forcing the Dark Gift onto Lestat, Magnus tells Lestat he is about to leave him and there are things that he must know. Magnus tells Lestat that he is now immortal and he would crave human blood. He warns Lestat to stop drinking the blood of his victims before their heart ceases to beat, lest he pay a heavy price. He warns that the sunlight and a blaze of fire can destroy his body but then emphasizes only when his ashes are spread will his life truly end. Once Lestat is informed the rudiments of what entails being a vampire, Magnus leads Lestat to the secret passageway to his treasure trove - the inheritance that Lestat can use as he wishes.
Magnus then lights a pyre he prepared for himself. He gives Lestat explicit instructions to scatter his ashes after the fire destroys his body, fearing he may otherwise return in a grotesque shape he didn't dare contemplate. Magnus bids Lestat farewell and leaps into the pyre, leaving Lestat on his own.
According to Alessandra, the "Old Queen" of Armand's coven in Paris, Magnus was 300 years old when he made Lestat. This act was forbidden by the Satanic cult of vampires, for it was Magnus' many years and making only one fledgling that made Lestat a vampire deemed by them to be too powerful to exist for one so new to the Dark Gift.
Magnus is seen again in Prince Lestat as a ghost, having designed a younger body for himself, however, he is shown to have trouble maintaining it, and often unintentionally flickers back to his older body.
He and Lestat reunited during this time, and he expressed regret for abandoning Lestat.