*a horde of skeletons that are rising from under the plains as it was once a battlefield looooong ago... but what (or who) is causing them to rise now? maybe one of the PCs is the descendant of the people that was in this war and thus the skeletons are going after him/her. (okay corny but the point is, skeletons, lots of them, from it being the site of a battle long ago). But if it is a "who" that is causing them to rise now, that could give you a second encounter tied in to the first where just the skeletons are met
Perhaps they rise up
every night.
Two armies are locked in a strange, oddly formalized re-enactment of their final day.
Every night, one hour after the sun sets, the skeletons claw their way up to the surface to resume their ancient battle.
They don't fight right away- first there's the matter of digging through the churned ground for rusty helms, bracers, weapons and breastplates. The hafts of spears and every single bow are long rotted away, so the archers make do with simple clubs or thrown rocks. Someone always has to track down an arm or ribcage that ended up far away from the rest of his bones, and ally and enemy alike will join in the hunt.
When they're satisfied that they're not going to find any more weapons or bones, the armies start to form up. This usually takes another half-hour or so. Sometimes- no more than twice a year- a skeleton is found to no longer have enough pieces to participate, at which point both armies pause to hold a final funeral for the redeceased, whose animus passes on. The leftover bones are claimed by the most damaged soldiers who remain, if they have need of them, or tossed into the torn-up ground for later use.
Lady Kahech mounts her skeletal horse (it's her fourth- the legs are lost easily, so about once a century she has to take one from a subordinate) and is surrounded by two wings of light-fighters and skirmishers. Her forces ambushed the others, long ago, and they move onto a low ridge. It doesn't conceal them very effectively, but on the day of the battle it was covered with trees.
Commander Sarjahn gave up the ghost (literally!) within fifty years of the first re-enactment; the dragonborn always loved to lead from the front, and his bones continued the tradition. His second, Lieutenant Sorthe, is more cautious; she has more of her original pieces than Lady Kahech now, and occasionally jokes about it as the armies are (pardon the term) assembling. She mounts the bones of her great shieldcrown behemoth steed near the center of the column, which begins to march along the ground where not even a cobble remains of the old road.
The ambush is sprung, as it has been thousands of times, and everyone who ought to do so acts surprised. Then they settle in to the bloody (in a manner of speaking) work. Particular tactics shift over time, with the slow and irregular loss of numbers, the total loss of missile weapons and most steeds. Echoes of strategic skill spark inside broken skulls, seeking new variations that might have somehow escaped earlier iterations of the carnage.
They fight until one army is destroyed, or until the sun's light begins to bleed into the sky to the east. The latter is most common- there aren't any tricks left, it would seem, and the two forces can stalemate each other with exhausting ease.
The whole thing would be a simple wonder, something to watch from a safe distance and marvel at, if the opposing leaders were just a little more locked into their pattern.
Lady Kahech and Lieutenant Sorthe have spyglasses, broken spyglasses that still work for their eyeless sockets, and they watch the battle with care, seeking advantage... but they also watch the surrounding area, sweeping the corroded brass tubes across the horizon from time to time in case there's something worth seeing.
And if they see you, they're going to send a cavalry force to collect you.
They want new ideas, a more modern insight, interesting tactics they haven't heard of before. Fresh blood.
Inevitably, the first group of collectors will arrive just before the second, and both forces will fight each other for the chance to collect you. If you run away under cover of this clash, you'll probably escape without much trouble.
If you fight and destroy both squads, that gives you a brief window of opportunity... but the Lady and the Lieutenant will see you do it. And then it's time for a truce. Within a minute or so, they'll have diced for first choice of your party (using Commander Sarjahn's knucklebones, naturally, even if the Lady suspects they are somewhat partial to his own side) and both armies will move out together.
They know each other very well, and they can work together just as well when they have orders to do so.
You might beat a few more of them, and they don't have bows or spells, but there are hundreds of them for every one of you. Their cavalry will circle and cut you off. Their scouts will hide if there's somewhere to do it- and they can wait beneath the waters of a stream or bury themselves if it will give them a chance to surprise you. They're extremely well-practiced at digging themselves up, after all.
And if you're caught, you've lost a day at the very least, delaying your eventual arrival... if you're lucky and smart and skilled and strong enough to flatter, argue, confuse, sneak, and fight your way out of bondage.
Good news, though! They can't just kill you and bring you back as undead tactical advisers! There's a ritual involved, and it takes time to set up. They'll lock you up in a prison of bones, fashioned from all the skeletons you managed to smash as you tried to get away, and those bones will sleep (uneasy, but quiet) through the daylight hours.
And then the sun sets, your jail will unravel itself into your jailers.
Better be quick.
