this one would start slow but oh would it get evil fast
Event Start location: Western Blight (If I remember my lore correctly this is the home of the WA...if I am wrong put that as the start location).
Every hour the WA will send out one blight anchor. This anchor will leave western blight and continue till it reaches unblighted land (note, in the beginning they won't be going far). they then plant, blight the area. This is repeated untill the event end.
Event End: There is a machine in western blight (one would have to be made and put in) This machine would first have to be contained (large construction project) then destroyed, once contained it becomes a T7 boss (yes I am saying T7 because I want this to be a major group effort).
The great thing about this event is you can repeat it, from other locations, and it will remain fresh.
Event would almost be cooler if you didn't tell people it was starting.