The Six
Also known as Strangers, The Six were 6 powerful creatures that spawned as part of the Warriors of Destiny event cycle during the Summer of 2008.
Their purpose, who they work for, and other key pieces of information are largely unknown at the time of writing (Summer 2008).
They spawned in the following locations:
Only one spawned in each location.
They were difficult to fight. Their attacks were heavily fire-based, and they could cause pet Dragons to go wild. They were sometimes capable of one-hit-killing PCs.
The fiction surrounding The Six was purposefully ambiguous as to which side of the conflict they were on. However, they were certainly capable of evil acts, as two of them spoke casually of the pros and cons of committing genocide upon humankind, and one of the murdered several people outside Despise.
They would, however, not attack directly players until attacked. However, their area effects indiscriminately injured player characters, and they hued red, a traditional means of noting that an NPC is evil or hostile or both.
For loot, they left strangely-shaped gemstones and A Magical Lockbox. Inside the box was loot of varying quality, and a Crimson Cincture. The box itself was very valuable and in fairy high demand among players.
They left no corpses. Some of them would smack talk, taunting the characters who had slain them, assuring them that they had not really won.
Sources:
http://town.uo.com/bnn/article_593.html
http://town.uo.com/bnn/article_594.html