The Tranquil Tower is a dark basalt keep at the bottom of the Valley of the Yellow Serpent. It looks sinister but apparently recent. It has narrow arrow-slit windows and a simple but tall door that is not visible from a distance.
Inside is a desk where the corrupted form of Olaus sits (worms in a yellow cape). The mechanisms are designed for a “protected” lethargy, with traps and dark magic. After Olaus’s defeat the group finds mysterious keys (diamantine, one with green jewels, one of iron) tied to the lethargy functions.