The Twisted Grove has a pool of water beneath it that feeds the roots of the trees and Wood-trolls there.
'Beneath the Twisted Grove is a pool of water. From this source, the trees and Wood-trolls' roots bring in their nutrients...and also their corruption. The waters of that pool are foul and poisoned.
'I have collected a few more of Longbough's tears and believe that they may purify this pool like they did the Gauradan-tainted one.
'Head within the Twisted Grove and pour this into the waters below.'
The Twisted Grove is south-west of the Eavespires.
Remmenylf asked you to go to the Twisted Grove and pour the Ent's tear into the pool beneath it.
Remmenylf is at the Eavespires, north-east of the Twisted Grove.
Remmenylf will be dismayed to hear that the Ent's tears did not purify the waters.