Whispermere guardian

The Whispermere guardian was a zombie dragon turtle that lurked near Black Tree Bend in the Whispermere of the Shadow Swamp of the Plane of Shadow in the early 1370s DR.

The guardian was installed there by Esvele Graycastle, a priestess of Shar, to guard the water route between the Shadow Citadel and the Monastery of the Ebon Dome. It would rise and try to capsize any boat that tried to pass Black Tree Bend (which looked no different from any other turn in the waterway), and devour its crew. However, although mindless, music soothed this savage beast: a particular haunting tune played on a dragonbone lyre would calm the zombie, causing it to submerge back into the murky depths.

Esvele gave the lyre and the tune to the shadar-kai ferrymaster Sithierel so he could safely pilot the Necreme through the swamp, bypassing the guardian. However, it may have been destroyed by adventurers investigating the deeds of the cult of Shar in the swamp in early Eleint of the Year of Lightning Storms, 1374 DR.

Background
Unusually, the Whispermere guardian has a neutral alignment when a zombie in v.3.5 D&D should be neutral evil. It also retains the dragon turtle's steam breath weapon and capsize ability, when a zombie should lose these. These issues and other mechanical errors are noted by John Cooper in a review and unofficial errata at the EN World forum.