While not ``dark'' per se (it's high fantasy of the highest caliber), C.J. Cherryh's Ealdwood duology (The Dreamstone and The Tree of Swords and Jewels) had such a depressing feel in its original version that the author re-wrote the ending in subsequent printings.
Not sure how I feel about that --- I found the originals _very_ moving (but haven't been able to bring myself to re-read them)
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