Aleister Mason is a vampire, an ancestor of Darien Mason from seven generations ago. He is a member of an obscure bloodline who gained their undeath by feasting on fruit brought from the garden of Hades by Bloodwing. After becoming undead they were bound to care for the trees that bore the fruit(meaning, they became addicted to the fruit) by keeping the soil drenched in blood. The bloodline, called Hmanaftera (Greek for "Blood Wing" but sometimes mistranslated as "Afterhuman") soon discovered that human blood could satisfy their thirst as well, but they still found themselves returning to the subterranean orchards.
Aleister became a vampire in an attempt to break free from the possession of Bloodwing's spirit over his body. Aleister was later magically imprisoned by Jeremiah Mason in Winterfell, but he was freed when the Mason family defeated Jeremiah. Aleister sought to possess a time-controlling artifact (the Retroactive Continuity Device) hidden under the Bloodwing Foundation property, and Darien sent Koen to stop him. Koen staked Aleister, but Aleister turned Koen into a vampire in the process.
When Koen used the time device to prevent himself from having fought Aleister in the first place, Koen was cured of his vampirism but Aleister rose again, retaining memories of the fight that was undone.
Aleister, like his descendant Darien, is both a Spark and a Mage. He discovered a process of fermenting the fruit of Bloodwing's garden into Bloodwine, which stabilized and concentrated its life-sustaining properties. He also specialized in clockwork inventions, including Blood Dolls, automatons with vampiric traits that use the Bloodwine as fuel. Aleister kidnapped Qli-3 and turned her into a Blood Doll, now called Gematria.
After the destruction of the Bloodwing Foundation and Jeremiah Mason's secret lairs, Aleister became the only known source of the Bloodwine, which Darien used to create his own Reanimation Serum. Aleister attempted to blackmail Darien as the only source, but Darien instead purged his body of Reanimation Serum, resulting in unexpected side effects.
Aleister's Blood Dolls rebelled and flocked to the reanimated Marcus Mason, putting his production process in jeopardy until one of Bloodwing's demon servants gave him a sapling from Hades's garden of the original Bloodfruit trees. In the face of a growing list of common enemies, Aleister reached an uneasy truce with the Mason family so he could revive his business and resupply Darien with ingredients for his reanimation serum.
Following violent seismic upheavals in the Winterfell region, the Masons lost contact with Aleister. It is presumed his secret lair was destroyed, but his fate remains a mystery.