Quicksword

The Quicksword "Enilno" is found in Ultima II in the city of New San Antonio, in 1990 A.D.. The strongest weapon available in the game, it is also required to complete Ultima II.

During the quest to undo the damage done to the timeline by Minax, the Stranger discovered that the sorceress could only be hurt with one, special weapon -- the Quicksword, "Enilno." Unfortunately, the owner of the sword, Sentri, had been thrown into the New San Antonio prison not long ago. The Stranger had to break into the prison and free Sentri, before he could purchase the sword for 500 gold.

With the Quicksword in hand, the Stranger was able to defeat Minax, giving her the killing blow in her own castle in the Time of Legends, repairing the damage she had done to time itself.

Trivia

 * As an easter egg, the Quicksword can be found in the Underworld of Ultima V: Lazarus.
 * "Enilno" is "Online" backwards, a possible nod towards Ultima II publisher Sierra Online.