You can't outrun them, you can't destroy them

[Anders]

I just watched "Q Who", the episode where Q introduced the Borg. Before they were made a laughingstock by Voyager, they were the perfect villains.

"You can't outrun them, you can't destroy them. If you damage them, the essence of what they are remains. They regenerate and keep coming."

Have you delivered a truly memorable threat to your players, one that really made them scared? What was it? Tell us their story.

[whswhs]

The most scared I have ever seen my players was in my second Transhuman Space campaign, where they met a young woman who was the prototype for the "graces" in Template Toolkit 2. She had no special combat capabilities, but she was extremely good at influencing people, and was trying to make a good impression on them so they would approve her parahuman type for general release. The current version doesn't have Memetics Talent, but with Attractive (+1), Charisma 1 (+1), and Voice (+2), it routinely gets +4 to reaction rolls (Good or Very Good), and Smooth Operator 3 gives another +3 to most Influence skills.

[RyanW]

I once introduced a monster called "the Nameless" to my campaign. Physically, they look somewhat like misshapen skeletons wrapped in dried, inside-out skin. They are from beyond the world (and the Annals of Creation do not record their name, thus their name), and so their physical forms are merely manifestations, the destruction of which is merely a temporary inconvenience.

What made them really memorable is that they appeared because of the party's caster botching a powerful spell. I never told them I had intended to introduce them all along, and that the spell was merely a convenient excuse.

[Kale]

I ran a modern day covert weird ops with a couple PCs who mostly used conventional weapons. One of the PCs was a Gunslinger with akimbo pistols. He was so highly skilled that he could usually shoot around foes' armor, etc so there was very little he couldn't stop. I had a recurring villain start showing up who was a powerful telekinetic with always on TK shields with no weak points. The PCs had no choice but to run every time they encountered him because they had no way of directly hurting him. Eventually the Gunslinger had an encounter with him underground and triggered an explosion that dropped a mountain on him, although that just slowed him down.

[Anaraxes]

In the early days of D&D (not OD&D, into barely AD&D 1e), our local group of groups had the "Howdy Doody" -- a small, annoying, creature that generally came in packs. They did d4-1 damage. They were AC 10 (the worst possible in those days), quite fragile and easily killed -- a fraction of one hit point, so even sharply insulting them might kill them. And they were quite terrifying to most PCs, because if you killed a Howdy Doody, even indirectly, you became one. Parties would do anything to avoid them.

[LokRobster]

Originally Posted by RyanW

I once introduced a monster called "the Nameless".../snip
What made them really memorable is that they appeared because of the party's caster botching a powerful spell. I never told them I had intended to introduce them all along, and that the spell was merely a convenient excuse.

That is the coolest intro, like Serendipity in game. The players thinking that their mistake made the Villain occur.
That seems like it’d be about impossible to set up on purpose, unless you’re very very patient with story development.

[AlexanderHowl]

In a Pathfinder campaign, I once had the PCs confront a small army of half-dragon medusa (the products of a green dragon getting it on with a village of medusa). Their father had been killed after getting every medusa in the village pregnant multiple times, so there had been five hundred of them born over ten years. Their mothers raised them to be the instrument of their vengeance against the killers of their father and, while the local army might have defeated one or two, they could not defeat them all.

Eventually, one of the PCs figured out a non-violent solution that ended up working. The kingdom would send the half-dragon medusa a small tribute, and fifty men for husbands, every year in exchange for their service in the defense of the realm. Before then though, they were nearly impossible to kill through violence, and the PCs only survived by running away faster than their comrades.