While everyone's on the topic of Living Dead, I think you can keep the theme and make it a really good skill.

For starters, you should become invulnerable once you enter walking dead, as in you take no damage. Second, separate walking dead from the room mechanic by simply just placing Doom on your character along with the walking dead status. You become invulnerable for the duration of walking dead, can keep the invulnerable even if the Doom gets cleansed.

Would it be OP? I don't think so. You still have to get full cured or you die. I think it's a pretty good high risk high reward. You could say healing to full is easy, but you don't know how many times I've died to this bad skill because healers couldn't top me off even though they knew it was coming.