Although the map is labeled the Pool of the Sunken Well, that's only to allow the GM to show this map to players, and not give away the encounter. Its actually the Pool of Dead Children.
A tamashinaki, which is a soul-less being, one of the oddities that could arise in the cursed reincarnation cycle of Kaidan. In this case a reincarnating spirit attempts to takeover an existing living being, through whatever reason the invading spirit fails to take control yet the original soul is driven away. An empty body normally means death to that body. However, sometimes an evil elemental spirit takes over called a Tamashinaki.
Soulless beings can learn martial class trades, though cannot speak and makes for a cursed evil being. In this case, this tamashinaki is named "Blood falls on the Snow", a name given to a serial killer who has kidnapped, tortured and murdered 24 children at this location. Their spirits have formed a center of negative energy at the pool location where all their bodies have been dumped.
The encounter uses the new Haunt mechanic from the Pathfinder Game Mastery Guide. In this case, after the PC party decides to rest here over-night... at midnight a freezing mist rises from the pool moving outward at 10' per round causing 1 hp of frost damage each round, moving outward a total of 100' (nearly the entire map). Those sleeping within the 100' range must make a Fort save. If they fail they remain asleep taking 1 hp of frost damage, while ice begins forming on their skin. They also dream through the mind of a child the chase, torture and murder of them on a frozen lake.
Those who awaken have the frost immediately melt off, and discover a thick freezing mist obscuring their vision. 24 advanced zombie children crawl from the pool moving through the mist to attack anyone they can catch. If three or more zombies grapple any person they will drag them back to the pool to drown.
They can awake and escape the hundred foot diameter reach of the center of the pool to escape attacks by the dead children. They can attack and destroy the zombies as well. However to end the haunt, the party needs to kill all the zombies and bury them outside the 100' radius.
This is meant to be haunted location 4/5's into the adventure.
Hand-drawing, digitally scanned, imported to Xara. Beveled shapes of mountains and ground topography with drop shadows, as well as drop-shadowed trees fill the terrain. My usual hand-drawn style.