
Dark Tree was an abstract term, a description of the growing threat of evil in Middle-earth - its roots can never be fully destroyed so that evil will once again arise if the 'tree' is left untended or unwatched. The term was used by Borlas in his conversation with Saelon in The New Shadow fragment.
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The term "Dark Tree" was used by Tolkien in his fragment of a story "The New Shadow", (Part IV of The Peoples of Middle-earth).
It might refer to the original sin or more likely the Morgoth Element.Without the domination of an incarnate greater will, a Dark Lord, man would for the first time ever , have been totally confronted with his own free will and choice,therefore giving rise to a "New Shadow" which then would be the inherent weakness, "fallen" character and "being bored with good" of man himself.