Sentinel

It is not exactly clear what the Sentinel of Skara Brae is. The Sentinel was never seen in its undestroyed state in Ultima IX. Judging by appearance it originally was a big statue standing in the middle of the city of Skara Brae.

The Sentinel helped to prevent the spread of the corruption of the columns onto the island itself, but it was powerless when the Guardian destroyed the whole island with a fire storm, killing everyone. When the Avatar later visited Skara Barea, the Sentinel told him/her the mantra of Spirituality.

Trivia

 * It is clear that the video of Skara Brae's destruction was produced before plot changes, since the Sentinel is already destroyed in it, creating a goof.
 * In the Dialogue Patch, the Sentinel was created by Horance.