OK, today's entry for the creative challenge is what I like to call the Tontine Bortherhood. This idea is borrowed in part from Lloyd Alexander'd Prydain Chronicles, although I don't remember the mechanics.
The idea is of a Brotherhood of Assassins or Holy Warriors, which as the members die absorb the strength of their fallen comrades. You could view it as a bit like Highlander, except the Brotherhood is not adversarial, but sworn to avenge each others' deaths.
Their powers are generally minimal. They are able to see their brother's killers and vaguely track them. Beyond that they are warriors often tasked with guarding a hidden temple or stronghold.
This is how the scenario unfolds. Through their normal adventuring, the party kills some of these heavily tattooed warriors. Then periodically more of these similar warriors show up for revenge They aren't morons, so they will try mass together in larger groups, rather than running into the wood chipper. The structure of when they will show up is a variable, as these warriors are often scattered across the world.
Now this is why this gets interesting. As they're killed they gradually increase in hit points, levels and strength. So as attrition thins their ranks to half of the total, they are twice as strong. One quarter, they are four times as strong, etc... You can see how a rather minor threat starts to snowball, especially with strength, which I usually calculate as 18/00 being the strength of two men, 19 of three, etc... Fudge it. Their hit points I straight out add and use a standard for all of them to make the math easier. The levels are even easier. Just make a page in your notebook to check off their deaths, with the corresponding upgrades in power. Fifteen minutes of math, you can lay out how strong the next guys will be and be ready for dozens of eventual fights.
So over a lengthy period of time, you have these groups show up and try to kill the party. It's a running battle, one which they might figure out or might not. The idea is to give your campaign a longer story arc, which isn't necessarily related to your other story arcs.
How large a brotherhood? I've done this with a hundred, two hundred and five hundred men.
Isn't that an absurd number for hit points for someone to have by the end of the scenario? No. There are ways to immobilize people without killing them. The party can always run away or run away some more. I usually do a quick estimate of how long till such a guy can steal a conveyance and come after the party, as they can't hide, even in other dimensions. The end game doesn't always have to be fighting one guy with a hundred hit dice and a titan's strength. It can be though. Remember that it will often start with group attacks, they'll attack when they think they'll have the edge.
How I've used this in the past.
The first time they were one of a number of guard types in a celtic temple complex in a distant land. The party flew in from out of town, stole an artifact, and skedaddled. I believe it was a largish alter, several tons, which would Cure Disease anyone within fifty feet. Which is a pretty cool thing to have, especially if you want to establish your own holy shrine and really bring in the pilgrims. This led to other problems of course. Some other clergies were a bit jealous, thieves were always trying to steal the tithes, etc... The party was off on other quests as well, the campaign had hit the point of everyone getting their own special quest for what they wanted. Meanwhile on a regular basis these guys with tats keep showing up and trying to kill them. They shrug off some of the attacks, but it eventually gets serious. Long story short, a guy ends up in a volcano.
Since then I've pulled it more than once.. Sometimes the last few guys get imprisoned, sometimes the campaign breaks up before the end game starts, and at least once attrition killed off everyone. As I mentioned, this is never the focus of the campaign. Some campaigns I use a variety of enemies, who's anger typically grows at the damn kids who keep interfering with their plans. Reoccurring characters who are villains are pretty sweet.