After learning that Arrod's bird was, in fact, alive -- though wounded -- you returned to Arrod, bringing Caledhroval with you.
'As I tended Caledhroval's wounds, he told me of what he learned upon his patrol. The situation is more dire than it seems.
'That trolls have begun to come down from the Misty Mountains has long been known, but we thought it only a few. However, it appears that there is an entire cave of them not too far from here. After spotting the cave, Caledhroval thought to warn Elladan and Elrohir, but was intercepted by the hendroval... apparently an agent of the trolls.
'Unfortunately, Caledhroval is not clear about the enemy's number. Would you scout the cave entrance and learn what the strength of the trolls' number is? The cave lies to the north-east. I shall await your return.'
The troll-cave lies to the north-east of Arrod's camp.
Arrod asked you to scout the entrance to the troll-cave, which Caledhroval discovered, and and see what you can learn about their strength.
The hendroval can be found near the entrance to the troll-cave in the Bruinen Gorges, north-east of Arrod's camp.
You should deal with the hendroval before it can warn the trolls.