One Arm is a rock demon that had lost his arm when Arlen Bales severed it when trying to escape.
After Aren moved to Miln, One Arm attempted to breach the the walls to kill Arlen, and after around one year One Arm found a weakness in the wards and broke through the wall, causing a breach, and he almost killed Arlen in the breach, but Arlen was cleverly able to avoid his attacks make it into the portable circle. One Arm would continue to test his wards for the rest of his life until Arlen was able to kill him with the Spear of Kaji on the outskirts of the ruins of Anoch Sun.
The wall around Fort Krasia was high and extremely well-awarded. However the sand-dwellers of Krasia had no experience with Rock Demons, and had put no rock wards on their wall. Also, their slingers weren't aware of the danger of slinging rocks at a rock demon. When the demon broke through the gate it was catastrophic for the Sharum in the maze. As in Miln, Arlen was instrumental in stopping the Demon's destruction and herding into a demon pit to await the sun.