Before they attacked, the raiders each laid a single stone in a great mound, planning that after the battle each survivor would remove a stone, leaving the cairn as both a tally of the fallen and a monument to them.
Three hours later, almost all lay dead in their foes' hall - the enemy slaughtered, their past dishonours avenged. The few survivors held a great feast there amonst the carnage, toasting the ghosts of the fallen.
The fest lasted but one night, for the loiving, but the ghosts of the dead continued their feast every night thereafter. The survivors removed their stones from the Cairn and, having now conquered the lands, settled in the villages nearby. Three hundred years on, many of the prominent locals tell of how their ancestors conquered the lands in a great battle; they point to the Cairn as a monument to the battle, and they never, ever go near the ruins of the haunted hall.
Then, very recently, the Cairn vanished. Or, more accurately, it was stolen - all very secretly, of course, with many a Perdo Imagonem to hide the deed. One of the local magi had investigated the cairn at last, and had discovered that each of the 100 stone contained a pawn of Mentem vis.
More recently still - just last night, in fact - the cairn reappeared (less 3 stones) - and the magus was found lying in his lab with a great scar (like an old sword wound) in his chest. He was still breathing, but his spirit was gone. A Criamon at the Covenant confirms that thi sis no Twilight, however.
At this point, a passing player character (a Quaesitor, preferably, or a respected member of any other house) is called upon to investigate the "murder" of the mage. None of his sodalis at the covenant trust each other - all suspect the others, none know of the cairn - and so would like an outsider to investigate. The locals are terrified of the magi, but will talk happily to other "visitors" (especially those not known to be attached to the covenant, and who don't "feel" like the magi - i.e. don't have the Gift).
What has actually happened is that the greedy mage has stolen the rocks for vis, but has discovered the hard way that each time he ues a "pawn" of vis from a rock, the ghost which is commemorated by the stone is destroyed (or bannished, or passes on, etc.). The ghosts in the hall, still feasting, were shocked when (after a season of experimenting) three of their fellows were "killed". They appeared before the magus, and "killed" him.
Problem. They cannot kill him - they have no physical form. He thinks that he is dead. (He saw them slice his chest with a sword - he must be dead.) So, they "take him hostage" (kidnap his ghost) and drag him back to the hall as a trophy, captive and (probably) butt of their jokes. He thinks that he has been dragged to some "pagan hell" for eternal punnishment, and refuses to even try to escape.
Now, the player chaarcters will have to work out why the magus was "killed". Talking to the locals (if characters don't have the Gift, remember) will reveal that the magus had asked, about three months before, about local legends - and of course he had been told about the Cairn. The local villages are full of the news that the "vanished" cairn has now returned. Getting the rest of the legend, the players should think to head up to the hall.
In combat, the ghosts infloict fatigue instead of body levels' damage (pain! their swords still hurt like hell!), and can seize the spirits of unconscious folk as further hostages. They cannot be Perdo'ed - the only way to destroy one is to "burn up" the pawn of vis in his rock. Moreover the magus believes that he is dead and in hell.
Getting the magus's spirit requires either great cunning, negotiation, trickery, or a huge waste of vis (Rego'ing the spirit back into the body is easy - getting the ghosts to give him up is very hard, especially given that they want to punnish him for slaying their three friends). Stealling the vis in the Cairn stones is a bad idea with a capital BAD.