The Imprisonment of Mastrius

The Imprisonment of Mastrius is a quest available in The Elder Scrolls III: Morrowind.

Quick walkthrough

 * 1) Speak to Mastrius in the Salvel Ancestral Tomb.
 * 2) Depart for Bthuand to locate Spellbreaker.
 * 3) Bring back Spellbreaker to Mastrius including a Daedra's Heart.
 * 4) Concede to help him out.
 * 5) Kill Mastrius.

Detailed walkthrough
The player must first speak to Mastrius who is located in the Salvel Ancestral Tomb. The Nerevarine must be a vampire to start the quest. Upon speaking with Mastrius the player learns that he was held against his will by Azura, and he thinks he has found a way to free himself. Where he then reveals that he needs the player to retrieve Spellbreaker and a Daedra's Heart.

After speaking with Mastrius depart for the Dwemer ruins of Bthuand and locate the shield in the inner more area of the ruins.

Upon returning to Mastrius he will ask for your help once more the strength to fracture the spell. This time assisting him causes him to absorb half of your health and fatigue, then turn hostile and attempt to kill the player. Once defeated loot his body, to claim Spellbreaker again which ends the quest.