Lord Blackthorn is the central villain of Ultima V. He also makes an appearance in Ultima IX.
Blackthorn's real-life counterpart is Starr Long, one of the long-time Origin employees, best known as the co-creator ofUltima Online. Blackthorn however was not modeled after Starr Long as he wasn't an employee of Origin at the time of Ultima V's release - he merely went on to play the character in Ultima Online.
Prior to Ultima V, Blackthorn was a close friend of Lord British, and known as a very virtuous and wise man. It came as no surprise then, that Blackthorn was chosen to rule by Lord British, while he would explore the Underworld. Once the Shadowlords captured Lord British however, they started to twist and poison Blackthorn's mind, making him cruel and evil.
He then codified the Virtues into draconian laws, erecting a horrible dictatorship under the influence of the Shadowlords. This was the state of things in Ultima V, when the Avatar arrived. Blackthorn tried to imprison Lord British forever, which he told the Avatar under the (false) impression that the Avatar had joined the Oppression (a lip-service done by the Avatar). Lord British was eventually freed however, and the Shadowlords destroyed. Blackthorn, now free of their influence, became aware of what he had done; he accepted Lord British's offer of exile rather than face the judgment of the Great Council, whom he had persecuted or jailed during his rule. Walking through a red Moongate created by Lord British's Orb of the Moons, Blackthorn vanished from Britannia to an unknown location.
It is known from a book on Serpent Isle that he lived with the Xenkan Monks for an undefined span of time, becoming a humble man by their accounts. He left the monks to go on his own travel before the Avatar arrived on the Serpent Isle. What happened to him after this is not known.
In Ultima IX, Blackthorn returned to Britannia as the Guardian's left hand (Most fans found this turn of events extremely curious, considering how Blackthorn apparently left the Monk Isle as a better person). His motivations are never explained in the story, though he appears to have an agenda of his own. He is ultimately killed by Lord British in the Abyss.