Masque of Clavicus Vile in Sorkvild's Tower - Morrowind
Location: Sorkvild's Tower in Dagon Fel
Requirements: N/A
Video: How to get the Masque of Clavicus Vile
The Masque of Clavicus Vile can be acquired as a drop from Sorkvild the Raven whose found inside Sorkvild's Tower. You can find this dungeon just east of Dagon Fel, the northern most town in Morrowind. The tower is visible from town and a very short run, use my map screenshot below if you need help finding it.
Inside the tower you will need to climb up a ladder which will take you to the room with Sorkvild inside. Sorkvild is involved in an Imperial Legion quest by the name of Sorkvild the Raven that requires you to come here and kill him, if you kill him before you get this quest the quest giver will thank you and give you the reward.
Note: Most of the loot in Sorkvild's tower is owned by him - if you loot anything make sure you sell it before any guards stop you for a crime otherwise they will confiscate it.
Sorkvild's Tower Map Location.