Grull was a city in the Shadowfell. The Tower of Grey were told to go there by a man named Tyson, in their search for Josail Longshadow1. It was a scary, prison-like place2.
It was later revealed that the entire city was one giant prison, which was inexcabable, and of which Josail Longshadow was the Warden3.
Eckhart Dayhammer once explained that Grull was divided into at least 4 levels. Level 1: The main prison, where everyone did their work, and ate, and slept. Level 2: The Guard's Quarters, where the guards lives and hung out between shifts. Also, prisoner Impound was located here. Level 3: The Super Evil Place, where monsters lived, and there were torture chambers, and was a bad time. Level 4: Unknown, though it was believed that Pelor and other important, powerful creatures were kept there.