Bone Crusher

In the early days of Pagan, Lithos, Titan of Earth, wanted the people of Tenebrae to worship him. The problem was that they already worshiped Hydros, Titan of Water. Enraged over this, he sent his undead army to destroy the city. Lord Elsmil the Tempest received Bonecrusher from Hydros, a hammer that could smite the undead. The army was destroyed in battle, and Lord Elsmil killed, but the Bonecrusher vanished. Until this day, the Bonecrusher hasn't appeared again.

Following a lead, the Avatar found a cave on the Plateau, where he found the legendary Bonecrusher, guarded by Invisible Stalkers. With this weapon, it was easy to overcome Lithos' undead creatures. It is unknown what happened to it after the Avatar left Pagan for Britannia.

Trivia

 * Bonecrusher is the strongest magic weapon in Ultima VIII. Its bonus is double the damage against the undead creatures (good, because there are a LOT of undead walking around).