So - if anyone's still trying to figure this out..
The archive is located directly under the Archive flag on the map - just dig. Takes ~20+ wit to spot it even if you're close.
To open the door you need to first learn a 'Song' from a book. There are two books, you only need one. One book is inside of the Demon Hunter's house (Jahna?) just south of the very center of the island, on the coast of the main land. Alternatively to the west of the Demon Hunter is a Witch who uses absurdly strong fire spells, but she also drops a copy of the book.
Once you know the song, there are 3 Angel statues on the island, sing the song to each and then purge the demon underground. One to the south west, one to the south, one to the south east. To do this, you need to...
1. Be able to see ghosts and pass a dialogue stat check thingy
2. Craft 2 silver levers to open the doors
3. Free the demon and kill it, cure it, whatever works.
To forge the silver levers you need to go to the north east corner of the island and use the ancient forge. 1 Silver Bar = 1 Lever. You find one bar next to the forge and one in the south corner's chamber underground.
