Phantoms

Phantoms are undead entities which are invisible to the naked eye, save for the physical arms and armour they carry. They appear in Ultima IV.

Description
Phantoms are said to be the remnants of dead warriors, who have been twisted by evil forces such that they must continually relive the battles of their previous lives. Their bodies, if they can be said to have any, do no appear to mortal sight, although they are easily detected by the mundane swords and shields they still bear. Other than the peculiarity of their cursed existence, these sad apparitions do not appear to have an properties which differentiate the from mortal fighters, but are fairly difficult to defeat without dispelling magic.

Phantoms were commonly found in dungeons and at other sites where men met their demise in battle. Appearing only at the very beginning of the Age of Enlightenment, these strange beings could occasionally in the company of the enigmatic wisps.

Trivia

 * While the unseeable spirits known as phantoms do not appear in later Ultimas, invisible entities such as Shadow Beasts and Invisible Stalkers may be found in Ultima Underworld I and Ultima VIII respectively. In addition to this, "haunted" areas in Ultima VII and Ultima VII Part Two would often exhibit strange phenomena, in which objects would shift and move as if manipulated by unseen hands.