During the quest to undo the damage done to the timeline by Minax, the Stranger discovered that the sorceress could only be hurt with a special weapon -- the Quicksword, "Enilno." Unfortunately, the owner of the sword, Sentri, was being held in the New San Antonio prison. The Stranger had to break into the prison and free Sentri, who sold him the sword for 500 gold. With the Quicksword in hand, the Stranger was later able to defeat Minax.
The Ultima 6 Project
In The Ultima 6 Project, Enilno can be found deep within dungeon Hythloth on the 3rd level in a lake of fire. Used to slay the dreaded Minax, this sword was also the key to returning her soul to Britannia in order to bring about the land's destruction.