Some players wonder - what eldritch abomination the DM will hit them with next? They don't worry about a horde of orcs - that's a numbers game. These poor players wonder what fiendish, obscure, horrible creature they will face next. A shambling mound? A will o' the wisp? An algoid? A phantom stalker? A dragon? A beholder? What is the most terrifying enemy a party can face?!
Players in my campaign know, without a shadow of a doubt, what the most terrifying foe they can face is -
An NPC 2-4 levels higher than the party average.
The NPC has all of the intelligence (or lack thereof) and motivation as the party plus he is evil, opportunistic, and higher level. I showed with Starbinder how a low-level NPC can totally disrupt a party just by being sly. Now imagine Starbinder being tougher than the party.
And the NPC doesn't have to be multi-class or a Bard to threaten the entire party. After all, he can have henchmen and hirelings, just like the party. NPCs can also have Charmed monsters, pets, etc. Better yet, have them all plus undead minions!
Want to see my 2e S&P party give a collective shudder of fear and revulsion? Just say 'pirate ship with a necromancer captain'.
Toss in a useful or creative magic item or three (I recommend they all have charges - after all, the party may get them!) and pow! instant terrifying bad guy!