Just hitting the high points with this post, as we spent a lot of time off-topic and spinning our wheels in debate - not that this is a bad thing, mind you, I'm actually thrilled at the amount of discussion the situation I placed my players in generated.
Ahmsi the Medicine Man, in gratitude for the work the PCs have done to free his village from control by the alien spores, offers them a brew of psilocybin, encouraging them to take a "vision quest" to see what the future may hold for them. Taking a shot apiece, the PCs found themselves in a world under a thick, leaden sky, the ground seemingly made of raw viscera, the sky patrolled by flights of fungal insect creatures and the land roamed by enormous, tree-like tripod creatures composed of gelatinous tentacles. They hypothesize that this is the world's fate if the fungal insects are allowed to complete their plans.
Studying the map they found, they decide to head in the direction of the Ettowah Mounds, which are marked with a very large X on the map.
Eventually the creature grabs a stick and starts making drawings in the dirt, trying to convey a story to the PCs. With a great deal of effort, Nashoba managed to figure out that the creature was telling them about men who came from across the sea in a great boat, and built the mounds, where they worshiped strange gods before being wiped out by the ancestors of the Native Americans. A great temple of these people from across the sea lies at Ettowah.
Arriving in Ettowah, they're stunned to see a little cottage among the mounds, smoke trailing from the chimney. Knocking on the door, Amity is stunned to discover her cousin Rebecca in the cabin; she and her husband Henry have come out to Ettowah, because Henry, an amateur archaeologist, is obsessed with proving that the mounds were built by the Welsh. He proudly shows the PCs a collection of rocks that he claims are covered with "Welsh Runes." Amity knows enough of history to recognize that he's full of shit and doesn't know what he's doing.
Come morning, he leads the PCs out to the site of his excavation, including showing them the remains of a shattered, trapezoidal door that he broke trying to lever out of its frame. Many of his "Welsh Runestones" were originally part of the door. Exploring the inside of the "temple" reveals a stone-walled hallway that ends at a cave-in. An alcove on either side of the hallway draws the PCs attention; both are filled with writing that Amity manages to recognize, astonishingly, as "Hyperborean," a language used 10,000 years ago in a lost continent north of Greenland. She can't read it, but she at least recognizes that it isn't Welsh. Investigating one alcove reveals a hidden door, that slides away revealing a staircase down into the darkness...