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.