If you have already acquired the Arroveins skull from the 1st level of the catacombs then go to Still water get the medallion from Beriand, Faeril might join u if u killed the dark nagas at the temple of mystra in the nightingale court and freed the good naga's temple at Windriders Glide. Then go to the Nightingale court and place the medallion to where the bad idol was and put the skull in the sarcophagus behind where the idol was then the arrovain is going to initialize a dialog, and teach you the Word of Necessity that u need to open that door in the Catacombs |