The Skull of Mondain is an artifact that is evil beyond measure. If used in an area with other living beings, they are all instantly destroyed by the sheer power of the evil within this skull. However, as it is with such evil, the price is high and all virtues will flee from the user of this dark skull.
In Ultima IV, the Stranger found the skull on the seabed between three volcanoes, at double new moons. The volcanoes would later form the island of Terfin, and Lord Blackthorn would build his palace in exactly the spot where the skull was found. This dark artifact was destroyed forever when the Stranger threw it into the fiery entrance of the Abyss during his Quest of the Avatar.
In Ultima VII, Erethian, an expert on ancient artifacts, states that the skull had been destroyed, which coincides with the events of Ultima IV.
In Ultima IX, an item labeled as the Skull of Mondain is seen in the Royal Museum. The truth behind the skull seen here is unknown, but it is one of many nitpicks some fans have with Ascension.