The Master's origin

From the book (Night Eternal, which is the final novel of the trilogy the show is based on)

Flashbacks to biblical times reveal the origins of the vampire race. The seven Ancients, including the Master, arose from Ozryel - the archangel of death. Ozryel was one of the three angels that God sent to destroy the cities of Sodom and Gomorrah for their wickedness. Ozryel was overpowered by blood lust while he destroyed the cities, being particularly enthralled by blood itself. When committing other atrocities necessary for cleansing the world he had not actually glimpsed blood before destroying these cities. Soon after, he betrayed and attacked Michael to drink his fellow archangel's blood. Appalled by Ozryel's actions, God punished him by having the other archangels cut his body into seven pieces and scattering them across the Earth. Over time, Ozryel's blood leaked from the burial sites and became sentient, spawning the Ancient Ones. The Master was the last to spawn, from Ozryel's throat.

Goodweather deciphers the Occido Lumen and determines that the Master originated on one of the Thousand Islands located in the St Lawrence River. The survivors detonate a nuclear weapon on the island, killing Goodweather, Zach, Mr Quinlan, and the Master. After the explosion, Nora and Vasily witness a reunion of the purified Ozryel with Gabriel and Michael, who had come to take Ozryel back to Heaven. The return was only made possible by Mr Quinlan, who brought the Ancients' ashes with him after following the instructions he read in the Lumen. After the Master's death, the remaining vampires disintegrate and the surviving humans are able to rebuild society.