Listener

A Listener is a creature summoned by the Guardian to spy on his enemies in other worlds. A Listener mostly looks like a cross between an imp and a bat and likes to be around lava and fire.

In Ultima Underworld II, the Guardian had sent a Listener to spy on the people inside the castle. The Avatar used a special weapon, given to him by Altara, to kill the Listener and thus reduce the Guardian's spying efforts. However, it later proved useless, since Patterson was his human spy in the castle.

Trivia

 * Killing the Listener is a vital point. Nothing else can be done until it is gone.