Blackthorn's Damnation was the name of an EA-run scenario in UO that ran November 2001 to January 2002. It was followed immediately by Plague of Despair.
This scenario furthered the corruption of the former good guy Lord Blackthorn at the hands of the vile entity Exodus. Exodus and Blackthorn altered the timeline of Ilshenar, and brought the Juka into the modern world to fight their battle against Britannia for them. The ancient enemies of the Juka, the Meer, were also brought back to fight alongside the players. It was at the return of the Meer that this scenario ended and its immediate sequel Plague of Despair began.
It is unclear if Blackthorn's Damnation existed in fiction only, and was merely a setup for the in-game events of its immediate sequel, or of in-game happenings overlapped the two scenarios.
The Scenario index on UO.com is extremely useful: http://www.uo.com/cgi-bin/newstools.pl?Article=6616