So, first, let's put a strong emphasis on 'fantasy' as in, 'fictional concept I enjoy' and not 'thing I plan to do to someone'.

But yeah, if you want darkest darkest, that's A) not something I've settled on, and B) likely would not be public information, but! Lately I've been on a slow-burn corruption kink sorta kick - wanting to take the role of a trusted confidant who gives advice and instructions that gradually warp someone into doing things that they never would on their own. Poison-tongued advisors to the throne, abusive therapists, corrupted artifacts, so on. I'd be blameless, of course - people would be suspicious of my influence, but my victim only ever speaks of me with the highest regard, and they're so forceful now that it's hard to challenge them. All with a healthy dose of hypnotic suggestion and conditioning!