This skill is the ability to recognize the trappings of the occult and deduce its purpose, or to create some trappings yourself. We don't judge. It can be as obvious as, "Hmm, the room of mirrors with a pentagram in the middle and bloodstains might not be quite right" to, "Neat, this scribble is the true name of the Duke of Hell, Galorigan". All occult knowledge would fall under the ban on magic in the laws of the Compact, and it's a perennial favorite of the Inquisition to describe in questionable detail the supposed practices of the accused who are no longer in a position to refute it.
For information about what your character should generally know if you have this skill, do 'help occult 1', 'help occult 2', etc.
0: Mirrors are edgy in Arvum. They go well with black clothing and poetry.
1: Alright, so, maybe not everything about demons is allegorical.
2: Sticks and stones may break my bones, but words can do way worse according to these texts.
3: You're starting to get a pretty good drift why some knowledge is considered forbidden.
4: Yeah it would definitely be the dark forces that want to devour everything. Those things.
5: You are all too familiar with them at this point, and ways to help deal with them.
6: And they are familiar with you.