Ether Gem

The Ether Gem is a gem capable of trapping spirits -- such as daemons -- within itself. This makes it particularly valuable, especially to mages.

Erethian originally possessed the gem, but Dracothraxus stole it from him (Erethian considered retrieving it, but in the end did nothing). In Ultima VII, the Avatar was told about the Ether Gem by Arcadion, who thought that it could free him from his mirror. Erethian however knew, that it would only transfer him to a more mobile prison. The Avatar retrieved it from Dracothraxus after defeating her for the first time, and used it to bind Arcadion to the Gem, fusing it into the hilt of The Black Sword.

The Gem remained in the hilt of the sword, but was damaged when Arcadion left it in Ultima VII Part Two. Repairing it with the Flux Analyzer, the Avatar used it to trap the Banes of Chaos.