I'm not sure exactly when he started feeling it - he was a healer for over ten years and mostly focused on the Scourge. But when it manifested in full, he -

[Deep breath. He needs that support, really.]

He couldn't die. The Scourge regenerated any harm done to him, and his aging froze him about as you see me now, save that his eyes turned yellow. I imagine he was also pained by sunlight, as most daemons are simply destroyed by it, but the immortality was the most prominent and worst symptom.

What it did to his mental state... I can't know for sure, but I can't imagine it was anything good.