Sand.
It was everywhere. Tiny grains rammed beneath his fingernails, crept between his eyelids and into his hair. It snuck like insects into his clothing, what little he had left, weaving into the rough fibers and scratching already sore flesh. His body was sore, his mind ached, and the heart which beat restlessly for freedom beneath his ribcage burned. The Arbiter's grounds was an accursed place full of torchlight and so, so much sand. The prisoners such as himself could hardly be called men anymore – he had become part of the place, breathing life into the stone walls, making the sand his own. It crammed into his soul and whispered sweet revenge against his burning heart, telling him he only need to wait. They were all waiting for death.
The sand promised revenge.
OoOo
In case you're wondering – I was writing this with the Death Sword in mind. I believe it used to be a prisoner whose soul was transformed into the monster after death. Just a thought