The Forgotten Ones is chapter 7, mission two, of the Legacy of the Damned.
When they arrive, Arthas proceeds down the hallway; however, Anub'arak stops Arthas, telling him that it is an ancient Nerubian trap and also tells him how to disable it. After disabling the trap by flooding the two corridors with water, they proceed and find a group of hostile Nerubians waiting for them. They free several Crypt fiends the Nerubians had captured, who join their cause.
The group continues, eventually stumbling into a Nerubian hatchery, where the nearby Queen forces the eggs to hatch and attack the intruders. Arthas and his men kill the Queen and manage to trap the spiderlings. Proceeding forward, they find a last group of Nerubians making a final stand, and break a gate down. On the other side are strange beings, which Anub'arak recognizes as the Faceless ones. They fight through the Faceless and several tentacles, freeing the remaining Crypt Fiends they had taken hostage.
After killing the Faceless Ones, they break down another gate, and find a Forgotten one. A heavy fight ensues, and the group barely manages to kill the creature. Suddenly, the caverns start collapsing and a large horde of Faceless Ones attack, prompting the group to flee.