This intended as an expansion for the Darklake chapter in the official Out of the Abyss adventure published by Wizards of the Coast set up to create random encounters for that area. But in hindsight it can be used at any larger lakesystem throughout the underdark with a Duergar presence.
The Guide is focused on two common types of locations found across the whole of Darklake - the Duergar built locks that enable boats to traverse the waters of this area linking the different ports of call together and the Deep Bells - an ingenious navigation system that allow boats to navigate the dark maze that is Darklake. Each location has a map with keyed areas along with a number of varying encounters. The Deep Bell section also has a navigation ruleset to allow players to make use of the Bells, taking shortcuts across the dark waters - a perilous undertaking if you’re not sure how to navigate.
- from the publisher's blurb