Pain

Most, if not all, acolytes who were present in the Grand Hall immediately turned their attention when Arcana's [Eye of Mistra] showed its might. It, after all, was a renowned skill that was both feared and hated by those who had experienced it firsthand.

Even with him trying to close his eyes, Thrax was still unable to get away from the Divine skill. After the bright light originating from the eye shone over him, the Immortal  Gladiator dropped the huge glaives in his hand and his defense was wide open.

Seeing such a sight, a haughty smirk returned to the child-looking acolyte's face.

"Hahah. you shall enjoy the pain."

The Divine skill brought those it entranced to an illusion of their deepest pain, which in Thrax's case was his past. 

It was the arduous time when he fought against the Roman soldiers who came back rushing at him like an unstoppable tsunami.