I'm guessing they're not and this is a bug? I'm doing the main quest, and I'm up to the "vengeance" bit where you go into Amaura and fight the Cur of Vengeance.

I discovered that the Tuatha were invincible, so on my way to Amaura, I just walked past them all, thinking this was normal. But when I got there, there was a fight between the Tuatha and my side, and you can't get into the House of Vengeance until you beat them all.

Plot twist: I went to a quiet corner so I could fast travel out of there and spent a few hours doing side quests, and when I came back they were no longer invincible.

So anyway, this got me into the building, but inside they were all invincible again. I started just walking past them again, because I didn't really have a choice, but this is problematic when I get to the Cur of Vengeance, because I need to defeat him and he seems to be invincible too.

Just wondering if there's something I'm missing or if this is a bug? I have tried all sorts of different weapons and magic, and none of it does any amount of damage. For the brief period that I could kill them, it was only taking me 3 hits or so to do it. The weapon I usually use is a greatsword with 180 or so damage, and my armour is good enough that I can stand around getting pummeled and taking lots of shots at them. I've literally stood there for about ten minutes attack one guy with everything I could throw at him to see if he would take damage, but his health didn't move.

tl;dr: I can't pass the Cur of Vengeance because he and all the Tuatha are invincible.