Plague of Despair
Plague of Despair was the name given to an EA-run scenario in UO that lasted from January to May of 2002.
In this scenario, the immediate sequel to the scenario Blackthorn's Damnation, the forces of Blackthorn and Exodus, together with the Juka race that had been literally pulled from the past of Ilshenar, attacked Britannia, especially the cities of Yew and Trinsic. (An attack on Britain was referred to in fiction, but does not seem to have actually taken place in-tame.)
This scenario and its immediate predecessor heralded the release of the Lord Blackthorn's Revenge expansion.
The scenario gets its title from the inadvertent effects of a spell cast by the Meer, which was supposed to drive the Juka out, but instead turned Yew into a swamp and unleashed a foul plague there. This plague is the source of the Plague Beasts and other plague monsters seen about.
The climax of this scenario has caused a great deal of confusion among players.
The result was different on each shard. "Winning" meant that Yew remained a swamp, but the plague itself was gone, monsters stopped spawning in Yew, and NPC Escort quests would recognize the city. "Losing" meant that Yew was still infected with the plague, and monsters continued to spawn there. (Many players think that it was possible to change Yew back from being a swamp, which it wasn't.)
Yew remained a swamp until the elves cleansed it, immediately prior to their return in the Mondain's Legacy expansion.
References
The Scneario index on UO.com is extremely useful: http://www.uo.com/cgi-bin/newstools.pl?Article=6616