Hi Teardropmina,
thanks a lot for your quick answer!
Yes, I used a two-handed Tenebrium-Axe, with no other "Extras" just 28 to 71 Tenebrium, 93 - 236 Hitpoints, 1 point of Tenebrium is needed for to use it (The character Madora, in my case, has 3 of that.)
I was at the bloodstone (right behind the portal) and when the char stroke on the bloodstone, it took some damage and then recovered pretty fast to full health.
When I used invisibility potions the demons said something like: come back into light out of the dark and saw me.
Thank you so much, trying to help me out!
Many regards!
Lemon