Blackrock Serpents

These three pieces of Blackrock, formed like serpents, are the key on the Serpent Isle to open the Wall of Lights in Spinebreaker, Skullcrusher and the Shrine of Balance. At the first two, any of the serpents can be used (it does not have to fit! Not only is this stated in the game, Batlin uses the Chaos serpent to access the Order wall). At the shrine of Balance however, all three Serpents are needed. The fates of these three Serpents is quite different.

Blackrock Serpent of Chaos
This one had come into the possession of the artist Andral in Monitor. Batlin stole it from him to activate the Wall of Lights in Spinebreaker. The Avatar took it after his death and used it first in Skullcrusher, and then at the Shrine of Balance, to open the Wall of Lights.

Blackrock Serpent of Order
This one had somehow come into the possession of a man from Pagan, who in turn died in a Goblin camp in Britannia (How? It's not known.). The Unnamed Goblin gave it to the Avatar in Ultima Underworld II as a thanks for killing the Bloodworms around a lake.

After arriving on Serpent Isle, a teleport storm exchanged it with Columna's stockings. Stefano of Moonshade, who had stolen the stockings, locked the Serpent away in his storage shed. Later, after defending Stefano from a Death Knight, the Avatar got the key for the shed and found the Serpent. He/she used it to activate the Wall of Lights at the Shrine of Balance.

Blackrock Serpent of Balance
This one had become the loot of the pirate Silverpate, who hid it deep in the icy north of the Serpent Isle. The Avatar found it after using the treasure map from the Inn of the Sleeping Bull. He/she used it at the Shrine of Balance to activate the Wall of Lights.

Trivia

 * The Blackrock Serpent of Order being brought by a man from Pagan indicates that the surviving Ophidians, servants of Order, entered through the Wall of Lights into the world of Pagan. Clearly another plot element being victim of the massive cuts made to the Ultima VIII's plot since Ophidians can't be found in Pagan.
 * One theory is that one of the Ophidians tried to seek help from other worlds once Guardian had invaded Pagan or was already under his rule. But the man in question was unable to focus his destination due Order Serpent being in slumber and unable to guide the travel through Void. Hence the unknown man was apparently randomly projected to the closest habitable world, Britannia.