The Archangel of Love, Love Mary [English] by [Satou Kuuki]
 

TG process is around 16 pages long and starts at the finale. Without spoiling too much , just gonna upload 5 pages of the process here before anyone proceeds to read the full manga here: https://nhentai.net/g/241018/

In short its about a magical girl, gets corrupted , who then proceeds to betray and TG corrupt her childhood friend (a magical boy) down to her own level.

Some may find the whole demonic corruption theme disturbing so beware.