So, I stumbled upon a Stormcloak encampment today while randomly exploring, and since I'm playing this character as being completely against the rebellion, I naturally attacked them. I wiped out all of them except for the named guy, Hjornskar. I took him to 0 health like 4 or 5 times, but each time, he just ended up kneeling on the ground, and after a few moments he would recover back to FULL FREAKING HEALTH. So, what's going on here? Why can't I kill him? Very annoyed atm.

The named commanders in both Imperial and Stormcloak Camps are marked essential. If you're on PC I think there are mods to change this.