Blackrock prism

The Prisms are by-products of the Guardian's magic.

In Ultima VII inside the three generators (Sphere, Tetrahedron and Cube) of the Guardian, there is a smaller copy of the geometric form of the generator, which acts as the central focus. Removing it destroys the generator. They have the form of a sphere, a tetrahedron and a cube.

The Avatar did exactly that to destroy all three generators in order to break the Guardian's hold on Britannia. Of the three Prisms, the Cube is the most important, because with it the Avatar was able to force the higher-ups of the Fellowship to tell the truth.

In the end, all three Prisms were used to de-activate the force field surrounding the Black Gate, giving the Avatar access.

Trivia

 * They should not be confused with Soul Prisms.