Misteln is Schariac's stigma incarnate, but jinxed. Cecilia did just that to heal most people from Honkai corruption and get Sirin's Cascade under control. She used her healing power to the extreme. Since Sirin's Cascade was a big anomaly in the Imaginary Space and Schariac stigma was heavily used, it was possible for Misteln to become the first ever "new human", i.e. a living stigma.