You’ll find Asra Nox when you reach the Ruin’s heart. If you lost a little health that you just want back, bring a weak slow healing item. If you lost some health that you want to get back fast, this is a good idea. medical kits are good for this, since they heal a good amount, and remove most negative effects. If you get to a low health level, you’ll want a strong healing item.
If you get to a critical health level, you’ll need something to boost your health, and fast.