The Spiral of Thorns is located inside Razorfen Downs, and is where the dungeon's most powerful creature, Amnennar the Coldbringer presides over his loyal subjects. Glutton the abomination is also found here, on the lowest level of the spiral.
Depending on whether the party takes a left or a right at the first fork in the dungeon, this area is accessed last or first, respectively. The only way to get to the top of the spiral is to follow its winding course around and around, battling the lich's undead minions along the way. The spiral has three full loops before it reaches the uppermost plateau.