This is what I did to get it.
- I began with all the points in Occultism
- I never drink ("More sober than Zadok Allen")
- I found the 12 occults items ("Strange secrets") -> Became master in Occultism.
- I accepted Drake's help ("Scholar of the unspeakable")
- I saved his ass ("Companion in arms")
- I read all the unholy books ("Complete revelation")
- I replied every time I could in R'lyehian ("Fluent in R’lyehian", I can choose that ending, reload and do the other.)
- In jail, I submitted to the Leviathan and I accepted the knowledge
- I killed Bradley
- During the visions, I refused to take the medicine, refused to eat the flesh but I killed the Dr.
At the end, kill yourself and pop :)
EDIT : I invite you to look the comments of Jotuns360Tag and Funky Monkee just below for an access to all the ends in one time.