The crawling chaos, Nyalathothep, has formed a cult on the mortal plane devoted to his cause which worships him as a god. This cult has been operating in secret for thousands of years, sowing destruction and chaos throughout humanity's history. He has sowed his taint into their genetics, providing a fruitful ground for many physical mutations among them. Considered gifts from their god, they have bred amongst themselves for generations, occasionally taking outsiders into their ranks to avoid inbreeding and becoming more corrupted and filled with his power. Now the time is ripe for Nyalathothep to enter the mortal realm, but for this, he needs a suitable host body, a perfect avatar, to contain his essence and allow him to walk the earth. This dark messiah would come to humanity as a living God and lead it to its next stage of evolution.
To be possessed by a god and being made one with him is considered the greatest of honors, one that all followers dream of being chosen as a host body. However, the cult believes that this avatar of Nyalathothep will be a human being lacking his genetic taint and unconnected with the cult. A being with a virgin template whose soul is uncorrupted and able to contain his essence.
"Chosen ones" are usually people who are connected with a religion or its culture in some way (Jesus, Mohammed, etc). For what reason would a religion develop the idea that someone outside themselves who have no real connection or awareness of them will be selected by their god?