Ancient One

The Ancient Ones are the group of peninsula inhabitants that built structures such as the sacrifice doors, obsidian wall frames, and obelisks. Though long gone from the peninsula, the traces they left play a major part in the story. Their structures are all made of the same material, which appears to be a type of obsidian. Most of their structures are only found at the furthest reaches of the peninsula, either deep below ground in the caves or way high in the mountains.

Known Background
Little is known about the society that created the structures mentioned here. A few assumptions can be made regarding the Ancient Ones, however. First, they pre-date modern technology and times. Second, they have some sort of mythical, magical power. This power has the ability to kill, to resurrect the dead, and to bring down planes.

Theoretical Background
It is quite possible that the Ancient Ones were a sect of druids. Druids were historically known for, among other things, using effigies, burning victims alive, holding secret meetings in caves and forests, having sacred trees, making human sacrifices, and believing souls could be transferred from one body to the next. In game, this would translate to the effigies, hanging corpses, the forest setting and caves, the sacred trees, and the resurrection obelisk. Either their attempts to use the resurrection obelisk went terribly wrong, creating the first cannibals and mutants, or they eventually fled the peninsula after discovery, and subsequent visitors attempted to use the obelisk, and due to lack of magical understanding, were cursed by it.

It is possible that the peninsula itself is built on a sort of ley-line, which is an invisible line of magic that runs through the earth. That would explain the bizarre occurrences on the peninsula (the obelisk powers, the mutations, the sacred trees).

Trivia
North American mythology, specifically the mythologies of the Abenaki and the Iroquois (both from the Northern sections of North America), reference cave dwellers and cannibals.