They are born of Cain's bloodline, which allows to be cloned, genetically engineered, or become hosts to entities, vril, and other parasites.
It's all about genetics.
This specific an innate gene either wakes up or remains "sleeping". I will not describe the method of gene awakening here, it has a great overlap into the occult level.

So say you're into genealogy....and you've traced many different lines far far far back....and many are of "royal" lineage...It has concerned me for several years now...because I honestly don't want to be associated with these bloodlines. At first I thought oh cool...not that it really means anything but..NOW...knowing the things I know about the elites and bloodlines and such. Uggh. I would like to believe that the blood of Christ crucified and risen again has truly made those of us in this circumstance NEW creations...washed in HIS blood and covered by HIM. Thoughts?