In the manor, there's some old books left from the previous owners, including an almanac with notes of the previous incidents. "August 11: Fields were flooded during the night. The rain was light but crops are wiped out. <Peasant's name> found a round green jewel in the field, about the size of a man's fist. We took it to the manor waiting for appraisal." That's the last log entry.
Local folklore, if pressed enough, also mentions said green jewels as a component of sorcery, but their actual method of use is not detailed.
On their first night at the mansion, the investigators can smell smoke and hear weird howling. Firewood has went missing in large quantities.
The foreman, Maunu Antinpoika, gets unrattled and starts trying to throw the investigators off the track.
Workmen start drowning. In places nowhere near water.
Finally the investigators see the fiend, a large humanoid-ish abomination that lurks beneath the sauna lodge, and wherever it goes, is followed by a filthy torrent of water it drowns its victims in. It has remained dormant for a century, but is now roused again from its slumber.
Turns out the green jewels are actually the creature's eggs, and local have been hiding them in other's barns, fields, homes as a way to "curse" them. The creature then would eventually awake and go retrieve her eggs, with terrible results.
Then the stories of the curse spread, the manor and the village were abandoned, and the creature could rest at peace. Now Maunu Antinpoika has rediscovered the cache, and being attracted by the pretty jewels, dragged them with him to the manor's basement vault to keep them secret and safe.
When the creature rouses, he realizes that the jewels are what the creature is after, and steals a large amount of firewood, trying to burn the eggs. Bad move.
The heat of the fire causes the eggs to hatch, something the creature's own, cold body couldn't manage to do. Now the cellar is full of little water fiends calling for their mother.
I expect this to end with the investigators deciding to burn everything.
