Mr. Fred. Mind Flayer with high stats, spell casting, and smart minions.
RID. Aka Ralph ID. AK Ralph the Intellect Devourer. In 1E I double or triple his psionic points.
Murr. Captain Murr. Started a just a random encounter in one of city state books. (Crazy man attacks party member does subdual damage.) Due to players throwing each other in jail became a fixture in the same cell. Then became addition NPC sailor fighter.
Grandmother Starr. Grandmother of one of my Kings. She worn combat boots. 8th level fighter who would sell npcs and pcs into slavery if they got on her bad side.
Green Thumb was he a rumor or not. GT could be a Sculptor of marble and gems. Or was druid medusa mix. Or was she an ancient green dragon with a huge range. The pcs never did find out.
Grr Plop. Red dragon with high stats and vorpal claws.
It was a while ago, so I can't remember her name right now, but she was a fairly standard archmage. She was pretty mean, though, and she had aspirations toward godhood. Through a series of events that was instigated by the PCs, she ended up with access to infinite Wishes, which she used to conquer the world in the distant past and become an invincible super lich.I am interested to see what sort of unique monsters and NPCs people have developed to vex their players, particularly versus higher level PCs. What nasty suprises did you build into their stat blocks? What sorts of schemes did they get up to? What were their lairs like?
My last mini-campaign ended around 8th level. The main villain was Harrumah the Long-Beard, an aged dwarf lord who was holed up in his fortress issuing demands to a xenophobic dwarven kingdom. The group eventually found that his fortress had been sealed from within for centuries and that the dwarf lord had actually calcified upon his throne over the generations. So they were hacking at a statue that was possessing the other members of the party and sending out ancestral wraiths to fight them.
I no longer run 5e "by the book," so my stats probably won't be much use for the OP. I do give my villains (even "mini-bosses") special abilities: auras, legendary actions, the ability to push/pull/slide/immobilize characters, etc. Just adjusting the numbers wasn't doing the trick for making them interesting or challenging.
If anyone is interested in the math or more specific tips, I'll provide them. Just don't want to derail the thread to much.