I just had a thought for your villain...
If the idea of the Easterlings (maybe the Balchoth) destroying the Elves in that wood sits well with you, maybe the forest could be inhabited by ghosts. Likely these would be ghosts of evil men who died in the battles with Bladorthin's (or whoever you choose) folk.
The ghosts themselves maybe present a suitable enough challenge to you characters in that wood, or maybe the ghosts are under the influence of a greater evil spirit or being. A Bandurhoth as was suggested...or an sorceror who's up to something in the woods...or even a spirit of some sort of your own creation. Maybe the ghosts are the spirits of the Elves themselves who have ignored the call of Mandos and still haunt their old home...there are many possibilities.
Use something that will provide a different type of challenge than the usual encounter. Whatever you choose to go with, I'd make the characters really work to figure out who's behind the supposed haunting of the forests.
Hope these have been of help.