The Knights of Thunder have not been forgotten. Their story does not end with the demise of Stormhold. The evil that befell their subterranean fortress has forever linked them to a greater tale that will be told. There are hints of where the tale took the brave Knights of Thunder.
As for the dungeon called Befallen, it is part of that tale and does exist in Norrath. I cannot promise you will see it again nor can I declare it will never be rediscovered. It still has secrets that would need to be revealed before we march to... well... to some place dangerous that I cannot mention yet. There are a few npc's that have great interest in the fate of Befallen and Stormhold. One of them has been encountered by a few of you lucky enough to encounter the spirits of the Peacock.
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