Hail, Lady Gretchen. Your question brings to mind a handful of experiences I have had. I have been struck by holy weapons of all designs and calibers, cloaked in holy fire or made from holy water. Each one breaks my scales and burns my flesh, but so long as I have souls to consume, I can recover. This has allowed me to develop a great resistance over time, as I no longer need to retreat to Hell to recover. I only have to feed if I have not done so already.

There are only two times where I have been unable to heal myself after being hurt. One was when the Blood Witch sank her teeth into my throat and filled my blood with her venom. I found myself paralyzed, in an almost dream-like state where I witnessed my greatest fears play before me on repeat. It fascinated me, but I was not able to recover until the venom wore off on its own.

The second time was when I attempted to seize control over your mate's mind. Something within him retaliated against me. Something powerful, and incomprehensible. It flooded through me, seeking somewhere to settle and, finding nothing, threatened to burn it's way into my very soul. As these were wounds upon my mind, the wounds upon my body they corresponded to did not heal for a long time.

I still do not understand what power it was, and until I do, my best defense is to avoid repeating my mistakes. I tried to make Halphas attack your son, and his resistance appears to have triggered the backlash I felt.