So in the process of doing the Satanist achivements.
1. Become Grandmaster of the Satanists.
2. Be the Child of Satan and then become the Grandmaster of the Satanists.
I took a queen all the way to the second highest rank only to fail at becoming Grandmaster because the Grandmaster refused to die. She did... a LOT of things to try and get there. She became publicly outed as a Satanist Queen and did her thing for years and years. She converted a Cardinal into the Satanist order, then made that Cardinal a pope. She sacrificed many dwarfs who may have also been babies. Also, luckily she got a quest to impregnate someone with the Child of Satan, so she impregnated her own daughter after breaking her mind and forcing her to be a Satanist.
Unfortunately, at the age of 57 she was killed by some demon hunters. But before she died she made her granddaughter, the Spawn of Satan, the heir to the Kingdom of Frisia.
A few uneventful years pass. I notice nothing really happens. None of the old Spawn of Satan events fire like the maid committing suicide or the witches or whatever. So I start wondering if something broke. Then I notice my government type is Reign of Terror. Oh yeah.
At 16, she immediately gets crowned by her Dark Father to become the Grandmaster of the Satanists, giving me both achievements.
And for extra kicks, I then used my Grandmaster powers to impregnate myself with another Child of Satan. We can't have enough Satanspawn after all.
I then asked the Pope (who's still the Satanist) to baptize my Satanspawn, but the holy water burned him.
Christianity is so fucked up right now.