During the reign of Julian the Apostate (360-363 CE), the Lasombra Marconius obtained a potion of what was said to be the dark god's blood from a mage and, upon drinking it, was transformed into the first of the Kiasyd. Some believe that this potion also contained the blood of Unseelie changelings. This deity's connection to the underworld explains why the original version of the Kiasyd bloodline (from the Storytellers Handbook to the Sabbat) have Necromancy as a Discipline.
The Vampire 20th Anniversary Edition expands on the Kiasyd origin: Vampire Twentieth Anniversary Edition: The Dark Ages has it that the Kiasyd were originally not a true bloodline but were instead created whenever a vampire Embraced one of the faerie-blooded. Marconius, however, claimed a different origin, saying he transformed himself using eldritch sorcery. According to The Black Hand: A Guide to the Tal'Mahe'Ra, Marconius' line eventually established itself as the modern Kiasyd bloodline at the expense of the other Kiasyd, who renamed themselves the Maeghar.