@black
There are mainly two ways to get the Mortuorati:
Through Russell: you have to have a good Influence over him (at the meeting with your counselors, tell Theresa and Juarez that you allowed him to use Mortuorati in Mordhaven; don’t be aggressive toward him; do not take measures intending to kill him). Then, ask him at the end of the conversation to teach you the Forbidden Art. If you have enough Influence, he will agree to it. A scene at a graveyard will be unlocked and Russell will teach you the ritual to obtain the Mortuorati.
Through Rose: this is a bit more complicated. You have to access the Rose RO pathway (starting at the convention center) and convince her over time that you are (mostly) a good person. When you meet her for the third time, at her home, she can teach you the Mortuorati.
No, the warehouse I mentioned is the Blazing Grave’s hideout. It encompasses a major part of the story after the ending of the “HUB part” (random scenes) of the game.