Palace of Blackthorn

Castle Blackthorn was a horrible place.

Errected on the volcanic island of Terfin after Blackthorn was manipulated by the Shadowlords, the castle was a dark place, fully of foul creatures, members of the Oppression and traps. The castle itself was intentionally built as a maze to cofuse intruders.

The Avatar entered it in Ultima V, to retivre the Crown. After the end of the dictatorship, the castle was left to rot. But not for long, as the mage Sutek made it his home, to conduct his experiments in peace. The Avatar met him in Ultima VI, as he/she passed through the castle, the find the balloonist with his plans. Shortly later, Sutek accidently turned himself to stone when fighting against some warriors. His statue was dumped into the sea, and the castle torched.

The Gargoyles used the stone to errect their own city, and so the ruin had vanished by Ultima VII, ending its history.

Trivia

 * Entering the castle without the Black Badge in Ultima V was a really bad idea...
 * Batlin still describes the ruins in The Book of Fellowship, which was written some years prior to Ultima VII, given a rough timescale, when the ruins vanished.