There may be more to the lore in-game, but the wiki states:
"Nictus are Kheldians who, when faced with the short native lifespan of the Kheldian people, turned to science and changed themselves into beings of darkness. They feed off of life energy, and were at one point able to siphon the life force of the Kheldians from great distances."
My point is that you could have a Kheldian that began the process to transform themselves into a Nictus, and either had a change of heart partway through, or it was otherwise interrupted, thereby only partially changing them. The player can then come up with whatever backstory they want to explain how they came to inhabit their current host...