When the resource rich but barren rock that Volantis was built upon was brought to compliance by the 9th none could have foretold of what the annals of time would wrought unto it's future populous.
In the final days of the compliance the Angels of Death were locked in a bitter guerrilla fight with the natives of the planet. A race of brutish humanoid mutants, who while technologically disadvantaged were more than well matched for an Astartes.
After Memnoch's squad had secured a strategic rocky over crop/vantage point they were ambushed. This ultimately culminated in Memnoch being buried alive in a monolithic land slide.
Believing him dead, his brothers and Primarch departed the planet.
Millennia passed and Memnoch still survived, dwelling under the rock, surviving on any nourishment he could find.
Eventually he succumbed, as do all his brothers, to the The Red Thirst but somehow Memnoch was able to coexist with this element of his nature.
Overtime the mighty Hive Volantis rose up around him and as the Aeon's travelled forward, it's sinking foundations forced Memnoch up into it's fetid Underhive.
No longer would Memnoch purely 'Survive', no he would truly 'live'.
Once again a demi-god in the presence of mortals.
A God King of the Dark.
And being as even a Vampire Astartes struggles with UV i present, The Harvester;