Dark Tree
From Tolkien Gateway
The 'Dark Tree' is an abstract term used by Tolkien in his fragment of a story The_New_Shadow (Part IV of The_Peoples_of_Middle-earth).
Dark Tree is a description of the growing threat of evil in middle earth - its roots can never be fully destroyed so that evil once again arises if the 'tree' is left untended or unwatched.
'Dark Tree' is a term used by Borlas in the The_New_Shadow in his conversation with Saelon.