The Ethereal Void is a place within the fiction of Ultima Online. It seems to be similar to the Astral Plane seen in Dungeons and Dragons, in that it is similar to outer space, but has breathable air.
It has been seen in the single-player Ultima games, but was not been seen in Ultima Online until the Mad Mage event (see next paragraph for an important qualification), when the Blackrock Detector created a mirror image of Moonglow in it, within which monsters were found, and from which the Shadowlords escaped into Britannia.
Though the Void wasn't specifically referred to at the time, in retrospect there's a good chance that the Void Shadow Dungeon was located there, as was a strange area that could be reached via the Magincia Moongate after the conclusion of the Magincia Invasion event (except on shards, such as Siege Perilous, that lost the event).
Despite their physical similarities, there is no indication (as of Summer 2008) that the Ethereal Void is the same as the Sea of Stars that surrounds the continents of Malas.