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. Unfortunately, none of his followers have proven suitable. Every vessel has rapidly broken down, unable to contain the essence of an elder god and forcing him back to his realm. What Nyalathothep needs is 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.
However, this avatar would not simply be the elder god taking over a mortal body, but someone else entirely. The original host and nyalathotep will both cease to exist, their souls combining to create a completely new individual altogether. This being may have the memories of its 'parent' souls, but feels no connection to them personally.
How can you showcase a complete change in personality and ethics and do it well in a story?