At higher levels, use Warlock Invocation Master of Myriad forms
That invocation let's you cast Alter Self at will. While it requires level 15, that is, quite a lot, it's not a disguise, you literally take on the shape you wish.
Recommendation: try talking to DM into allowing the use of this invocation at level 5-7, since the spell is only 2nd level and doesn't really give any combat advantages (you can negotiate locking out the Natural Weapons part of it until intended at lvl 15).
As another option, if you're only interested in roleplaying the intimidation, and not interested in getting a mechanical boon on top of it, I would look into trinkets.
Any character is allowed a free trinket, and Curse of Strahd provides Gothic Trinkets table that contains some quite magical items. If
a little black book that records your dreams, and yours alone, when you sleep
can be a trinket (and an actually useful thing, in case of prophetic dreams), then I don't see why "A ring, that makes your visage horrific for a second when you press the stone in it" can't be an option.
Unfortunately PHB doesn't really mention "you can think up your own trinket", so some will argue that it's illegal, but really, nothing is illegal if DM allows it. Work with your DM on this.