Celeborn asks you to close the many winding passages created by Ungwetári's spawn under the Mountains of Shadow.
'Beneath these mountains, there are many sprawling tunnels carved by Ungwetári and her brood. If we come across any small exits, we should close them to pen the spiders in. The Gilgelair will block the larger passages.'
Many small passages exist within Torech Ungol, allowing the spiders and gredbyg to come and go.
You should block these small passages to trap Shelob's brood.