Sorrow Hill Crypt is the dilapidated crypt on the eastern edge of Sorrow Hill is inhabited by skeletal magi and Scourge warders. Judging by its proximity to Andorhal, it is believed that this was the crypt that held the remains of the necromancer Kel'Thuzad after he was killed by Prince Arthas.
At the very bottom of the crypt is a small office. A desk, a bookcase, a few bookpiles, etc. Perhaps it is a necromancer's laboratory, or, as suggested above, the tomb of Kel'Thuzad.This section concerns content exclusive to Cataclysm.
The Crypt on the edge of Sorrow Hill is now devoid of mobs, but still accessible.