One of the best villains I ever came up with was only ever supposed to be a nameless Random Mook #5, a lump of hit points to be chewed through, but due solely to the emerging circumstances during the game - and without any real intentional effort on my part, lol - the party inadvertently turned him into the primary villain of the campaign...
(Or, at least the primary face of the bad guys, since this guy was pretty much the embodiment of the "Uber-competent Henchman" trope.)
In one of the very first fights of the game, bad tactical decisions and some lucky/unlucky rolls allowed the nameless thug to escape, taking with him something the party was looking for and warning his employers. I made the mistake of having him fire off a jaunty remark as he made his escape, which apparently really stuck with a couple of the players... When the party got to the point of confronting the people he was supposed to be working for, one of the party members specifically asked if the guy was present. During the ensuing fight, a PC's attempt to go after the guy backfired badly, and he once again escaped in a stylish fashion.
Even a level later, with a new villainous organization to go after, the party was still talking about tracking this guy down, so I decided to replace one of the assistant assistant head mooks with this guy (using a different name) and partially blame the party's lack of success on this guy's interference with their efforts. The party pretty much lost their minds when they found out their old pal was working with their new enemies, lol. And once again, party choices and dice rolls led to his escape even after the new organization was destroyed.
Fast forward to a few sessions later, and this guy has now become the party's eternal nemesis despite the fact that this guy doesn't even have an actual name or any personal motivations other than "Because... henchman."
They were completely uninterested in anything other than chasing this guy down.
So I just decided to roll with it, and had the party discover him working with pretty much everyone they were ever up against. The guy just kept upping his evil henchman cred as the party became bigger and bigger heroes, and even after he got to the point where the party would routinely find out that he was actually behind the groups they were fighting, it would always turn out that the guy was just working to fulfill the shadowy agenda of someone bigger.
Over the course of the short campaign, this guy had pulled all sorts of shenanigans on the party, including:
After being caught in the party's rooms stealing stuff, he led the whole party, clad only in their nightwear, on a wild chase through the back alleys of the city, only to have the female paladin (doing the whole Arthurian/Joan of Arc "paragon of purity" thing) burst out onto the main street in time to tackle an "old woman" out of the way of a runaway wagon. Said old woman was actually the bad guy in disguise, who kissed her on the lips and whispered, "Nothing personal, love", before jamming a knife in her ribs and leaving her to bleed out in the street dressed only in basically a t-shirt...
He framed the party rogue for a murder and several other crimes.
He'd nearly gotten the cleric excommunicated from their Church.
He stole the wizard's spellbooks and burned them in front of him (he'd actually replaced the books with fakes, then stolen the fakes and burned them, lol).
The game ended up breaking up eventually, so we ended it with the party capturing this guy and seeing him hanged.
The funny part?
Not only did the party never find out who this guy was actually working for...
...They never even found out his name.
(And I hadn't even given him one beyond the identities he'd taken on.)