Blackthorn is a great, tragic villain of Ultima Online. His name is very often misspelled to "Blackthorne," but "Blackthorn" is the proper spelling.
Originally, he was a friend and ally of Lord British. The two maintained their friendship even after Blackthorn founded the philosophy of Chaos to counter the dominance of Lord British's Virtues. While their followers sometimes slaughtered one another in the streets (through the Chaos and Order system, now defunct), British and Blackthorn continued to play chess and debate philosophy.
However, when he did not immediately ascend to the throne upon British's first absence, Blackthorn became bitter and resentful. Despite the tolerance and egalitarian sentiments of Chaos, at heart Blackthorn was an aristocrat, and thought, simply by right of noble birth, that he should have the throne until British returned. He succumbed to the temptations of the villain Exodus, and became true evil. He also became a half-human, half-machine monstrosity (with an obvious influence from the Borg from Star Trek), and the subject of a toy designed by Todd McFarlane, the noted comic book artist, that was included with the box for the Lord Blackthorn's Revenge expansion. (See the pages on the Gargoyle's Redemption, Blackthorn's Damnation, and Plague of Despair scenarios.)
At a memorial service for Blackthorn, Lord British stated that it was Blackthorn's death that caused his (temporary) return to Britannia. And further stated:
"My friend Lord Blackthorn is dead, and I am sorrowful at his passing. He became corrupted by great Evils, and now is no more. Though we were often at odds, the truth of his downfall wears upon my soul."
And also stated that, as a monument to Blackthorn's ideas, he was abandoning his goal of uniting the Shards, under the Virtues.
Thus, in death, Blackthorn effectively won the Chaos/Order wars.
On both facets, Blackthorn's castle is still accessible, gathering dust, used occasionally for roleplayed villainy, and standing as a monument to how easily Chaos can become Pride. He also has a castle in Central Ilshenar (from when he turned evil).
Here is a story on Blackthorn's memorial service: