Nobody has ever survived the night in the Three Valleys Bog.
Before the attack, the Heroes of Tara discovered standing stones. In fact, they discovered a number of them, in what was later determined to be a semi-circle.
Each set of stones were carved with the cyclops princess Spyridoula and her devotions to her hounds. The monoliths formed a circle, with cross members linking them to a central pillar and a statue of an angry cyclops, it's mouth open in a roar. It was determined that hot air was venting out of the mouth of the statue, smelling of peat.
As the party searched further, the discovered additional standing stones every few miles, usually situated in an especially boggy location.
As the undead roamed the land when the sun fell, something else peculiar was going on. One in five of such hordes was a work party. They would dig up peat moss in big clumps and leave it to dry, but also taking dried peat from what was likely previous evenings. They all headed back to the center of the circle formed by the standing stones. Here they heroes discovered some sort of funerary building.
Additional undead could be seen coming and going from the building, including quite a few "free willed" undead, like spectres and talking zombies and skeletons (lords more than likely). Atop the building was peat and other bog debris, dripping down the side, as if the structure had just emerged from the bog itself.
This is more than likely the Tomb of Spyridoula, legendary guardian of the cyclops people, who they say gave her life to defend the island from revolting humans.