Glass Sword
The "ultimate weapon", it can kill any target with a single stroke - Though as it is not crafted of the most durable material, a single stroke is all it is capable of.
Starting with Ultima V: The Warriors of Destiny, Glass Swords were rare weapons kept in reserve for the tougher battles. In some instances, they were the only method available to slay certain creatures. When forced to fight the Shadowlords, a Glass Sword was the only weapon that could temporarily incapacitate (but not slay) the foul creature.
The Glass Sword currently only appears in the Ultima Online demo (included with The Second Age), where the player is handed one in order to slay a Dragon.
The effects of Glass Swords were easily seen when the 2008 on-line event of the Royal Council Massacre was investigated. The Council Chambers were not only littered with dead bodies, but also with shards of glass. Assasins breached the Council Chamber's security and evidently slew the members with Glass Swords.
That item was introduced into production shards with the expansion "Stygian Abyss" and can nowadays be crafted by skilled blacksmiths or be bought from npc Blackmith gargoyles in Ter Mur.