The Soul Cage was invented by Mordra to trap the Liche in Horance, so that the Liche Potion could destroy him. However, Trent, the blacksmith of Skara Brae, died in a fire before he could finish it. His ghost continued the work in Ultima VII, rather fruitlessly until the Avatar helped him to overcome his rage. When it was finished, it was bathed in the Well of Souls. Now empowered it could be used on Horance the Liche during the Black Service to trap him.
It was destroyed, after the potion took effect, dissolving into dust.