Quicksword

The Quicksword "Enilno" is found in Ultima II in New San Antonio. It is the strongest weapon in the game, and also the only way to finish it.

During his quest, the Stranger found out in several conversations that Minax could only be hurt with one, special weapon, the Quicksword "Enilno." Sadly, the owner of the sword, Sentri, had been thrown into the prison of New San Antonio 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 hands, 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 a inside joke, the Quicksword can be found in the Underworld of Ultima V: Lazarus.
 * "Enilno" is "Online" backwards, a possible nod towards Ultima II's publisher, Sierra Online.