I am just one Hell of a Buttler
Seeing what you are looking for I would suggest the way of the Open Hand Monk or the Long Death and be a Tiefling as your race.
The Open Hand allows you the better monk abilities, your Fury of Blows gain the prone property and the Quivering Palm at 17th level is deadly and shows the lethality of your hands.
The Long Death gives you the more "devilish" aspect, allowing you to never die, as long as you have ki points. To give others around you the frightening condition, and your touch of death will keep you standing for a longer time.
Your "eldritch or hellfire" ability can come from the Tiefling innate spell casting, of which you could take the magic initiate for a few extra spells for the flavour.
But in my opinion, it would be better by being a College of Whispers bard, and if you want multiclass into Warlock.
The College of Whispers gives you Psychic Blades (Use of your bardic inspiration to deal extra damage.)
When you join the College of Whispers at 3rd level, you gain the
ability to make your weapon attacks magically toxic to a creature's
The use of Words of Terror reminds of how Sebastian can spread fear with his words.
Being a bard also give you proficiency, it fits well for a Buttler of the Phantomhive household, he excels in all his task. The Bard is a spell caster so you can get your demonic powers from those spells.
If you wish you could Multiclass the Long Death and College of Whispers to get the best of both.