Hiya!
Good to see you're enjoying my game!
As for the name of the cultist leader: while in the Crypt, there's a clue in the book (which you seem to have found):
'I found a way to open the door. they asked me my name. I felt my end approaching. but then a writing came to my memory. The one in the north...'"
The last line refers to a north(ern) room. There are two northern rooms in the game, you'll have to go to the North Room close to the prison.
There's a book you might want to check...
Good luck!