Der Fluch des Flussvaters (2012)

from the back of the book

Havena, gateway to the sea of the seven winds at the mouth of the Great River, and the capital of Albernia, a hoard of ancient secrets. For centuries, the struggle of the forces of the sea god Efferd and the archdaemoness Charyptoroth goes on. Unbeknownst to the citizens of trading port who after years of war with the Northern Marks only want to restore prosperity, the shadows of the sunken undertown rise again: people disappear and ships are attacked by monstrous abominations. Even the Father of the River, Efferd's governor, who is said to have his court in the undertown, takes little notice of what is happening.

To rouse the Father of the River and avert the demon threat, the heroes must dive into the underwater world - because even in his court is undermined by enemies.
On the hunt for the henchmen of the Daughter of the Deep, the people from Thorwaler offer a real help. But what is their real agenda? Why are they so eager to break the old curse of the Father of teh River, who prevented for 2,000 years that Thorwal people travel the Great River? Finally, the heroes must make a decision about how they want be remembered by the bards: as friends or enemies of the northern people?