'Hoom! I must learn more about the power which has corrupted the Twisted Glade and seeps into the surrounding forest. Return there and gather any sickened or dead plants you may find. From them, I may be able to determine what it is that we truly face.
'Hom! Ta-room! As you well know, the Twisted Glade is beset by wood-trolls of great strength, you would be wise to bring allies with you in your search.'
Longbough believes that some evil power may be twisting the wood to dark ends.
Longbough has asked you to seek evidence of the power which has corrupted the Twisted Grove. He asked you to collect rotted limbs and bring them to him.
2. Longbough is at the Eavespires, on the northwest shore of Lake Evendim.
You should bring the rotted limbs you have collected to Longbough.
Rotten limbs may also be found on the various wood trolls in the area. It seemed like a rare drop, though.