A novel with ‘Master’ in the title that has a dark/gothic tone and possibly includes satan, or at least has satanic elements (not ‘The Master and Margarita’). Published around 1900-1970.

[Steerpike]

A less well-known piece by an English or American author, probably written sometime between 1900-1970. I was convinced the author was Mervyn Peake but not in his bibliography. Title may be something like ‘The Master of (a place/house/estate)’.

[Amber (Daisymau)]

This book?
The Master of Blue Mire by Virginia Coffman

[Orsolya Könyvtáros (formerly Memp)]

The Master of Ballantrae? Very dark, lesser-known work of Stevenson's.

" 'He's not of this world,' whispered my lord [...], 'I have struck my sword throughout his vitals [...] but he was never dead for that,' said he, and sighed aloud. 'Why should I think he was dead now?' "

edit to add: Stevenson said it contained "all I know of the devil."

[Kris]

Wikipedia's detailed plot summary - with Spoilers - of The Master of Ballantrae by Robert Louis Stevenson (Memp's suggestion):

https://en.wikipedia.org/wiki/The_Mas...

Complete text on Project Gutenberg: https://www.gutenberg.org/ebooks/864