Gem of Immortality
The Gem of Immortality was a magical artifact that made Mondain nearly impossible to defeat. The Avatar defeated Mondain by shattering the gem.
Ultima On-Line
Once the gem was shattered, divergent realities were instantly created- one reality contained within each shard. One reality continued on its course, this became known as Ultima Prime and is the setting for the single player games of Ultima. The other pieces housed realties or Shards of UO upon which our characters all live.
When Lord British discovered that the gemstone shards could be used to traverse realities, he gathered as many pieces of the Gem that he could (presumably not the same ones the characters all live on) and secured them away in the Ethereal Void to guard them from Minax’s evil uses.
Ultima Lore
Ultima VI: Warriors of Destiny details how three shards from the ‘’’Gem of Imortality’’’were lost and forgotten, buried deep within Sosaria. After decades of safe keeping, the shards were unearthed by the same seismic activity that created the Underworld when the Avatar removed the Codex of Ultimate Wisdom from the depths of the Abyss.
Captain Johne and his crew aboard the Ararat were pulled to the underworld through a whirlpool. Capt Johne discovered the trio of shards, and he was instantly possessed by the gem. They compelled him to kill his remaining crewmates. Through the malign forces of the gem, and the spilled blood, the Shadowlords came into existence. One Shadowlord for each gem shard
These shards (and Shadowlords) were eventually destroyed when the Avatar cast one each into the Flames of Truth, Love, and Courage.