The Malledhrim have asked you and your companions to enter Barad Guldur in an attempt to strike at the heart of the forces of Dol Guldur.
Barad Guldur is the mighty tower at the fortress of Dol Guldur.
Once inside, Belvaer has asked you to sever the flow of enemy reinforcements streaming from the tower. Belvaer seems to think that the captain of the guard should be found soon after beginning your ascent of the tower.
Anglú is in Barad Guldur.
You should speak with the Elf-lieutenant Anglú for further instruction on your offensive.