I was in Markarth when a Stendarr Vigilant attacked me without any reasons. I tried to kill him but it appears he can't be killed, so every time I visit Markarth I get harassed by this moron. Is this because I summoned a Dremora Lord or is it part of a quest. I never had him attacked me before. He is the guy who stands in front of the abandon house.

Cant say for sure. Ive only been attacked by them if i transform into a vampire in front of them.

Makes perfect sense. So would i.

it may be that if you are consorting with Daedra they might also attack. Just a guess.

Nah, I've approached them many times with a daedric summon active and they've only complained about how my spell "looks dangerous."

Sounds like a glitch to me.

I think you are correct. I've had this happen before for no reason, but I feel like everything was fine when I went back to Markarth. I did have to drop him to his knees and then run away to get out of there, though.

TC, if your Illusion skill is high enough, you could try Calming (or Pacifying) him and starting his quest. You don't have to complete it, and it'll get him out of your hair once and for all.

Or, if you're on the PC, you could try fixing him through the console.

I've also been attacked by them for no reason multiple times. Not sure if it is some glitch though or if that's just how they are sometimes.

I've never been attacked by them, probably because I attack them first!

When I broke out of Markarth with my forsworn pals he decided to stop us. Its to bad he couldn't be killed. Same with Durak. We beat the crap out of him which resulted in a bounty in the Rift for me.

Wonder if you have some kind of bounty the other guards ignore but he doesn't?

Doesn't the Vigil of Stendarr has a HQ in Dawnstar?

I had exactly the same thing happen on one play through.