Strontius had not known pain to this degree before. He had been slashed, stabbed, ripped apart, sundered, burned, shot at, impaled, crushed, drowned, and flain alive in battle and torture before. But this... his very *essence was being ripped apart and tainted. He could feel his very veins burning as the Fel-crystal prison around him seethed with pure demonic energies that he had not tasted since he had forsaken all magic except holy. His body was reacting violently to this now, bones crunching, his skin was changing... he roared inside the prison, and started pounding at the walls as his skull pounded and felt like it was splitting apart...
Finally, he felt his knuckles shatter the prison, but as the evening light shone in in the desolate wasteland, he could see and feel the horror of what happened had just begun.*

This is non-cannon for this character, the stalwart Guardian Strontius Sunspear is still fine, this is just a scenario as to how and what could happen to turn him into a monster.
The skilled Faint had done this for me in a stream, rather pleased with it if i do say so myself!