Perhaps because a number of the events that were cut from the original chronicles had quite a dark edge. Either that or dark could also mean in the same context of the dark age as it is period of the chronicle that was lost to knowledge of sorts.
Anyway, it sounds like a pretty interesting work for them. It'll be interesting to see how well all of this will mesh together. Not to mention the new characters to be introduced in the trilogy and the novella work in additional this series that will feature those characters.
Pondering the Mysteries of Dragonlance
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