Haugbui

The haughubi are a type of undead person who is confined to an area close to their barrow. They have most of the powers of the Draugr; except those that require them to leave their barrow. Therefore, they cannot turn into a cat, for example, and attack someone in their home.
Haughubi are typically buried closed to the sea or a large river or lake. They have the ability to shape change into a sea creature that resembles sea weed and float under a boat, even envelop a small boat and drown those aboard. They may, alternatively, appear to be in a small craft that follows under the water or swim alongside. They may take the form of moss covered stones along the shore to ambush and drown an enemy who invades their territory.
Haughubi are also immune to normal weapons.
They are also limited to becoming three times their normal mass and strength.
They may be destroyed in the same manner as a draugr.