In Morrowind, it was possible to permanently kill NPCs who were involved in the main story. This would result in a message basically informing you that you had broken the world and to reload a save if you want to make progress. In Oblivion, this was changed so that important NPCs were "knocked out" instead of killed.
In Skyrim, is there any (outside of console or game file editing) way to permanently kill an important NPC or can I go on a killing spree relatively care free?
Important NPCs are invulnerable, like Oblivion.
You might close off some smaller sidequests, but the folks necessary for the main story (for instance) have a tendency to be "incapacitated" from "killing" blows.
So feel free to go nuts.
Interestingly, NPC allies behave a little different, even if they are non-essential (and thus ripe for killing). When they get reduced to below 0 health, they revert into the "yield " state, which usually means enemy NPCs stop focusing on them to come after you instead. If they get hit while in this state, they will die.
Additionally, you cannot put them into this state - your attacks will directly kill them.
You can't permanently kill important people, they only become temporarily incapacitated when badly hurt. I just confirmed it on the XBox 360.
I was walking along with an important NPC and we attacked a Giant. The Giant practically one-shotted him and kneeled down in agony while recovering.
So while they aren't blown unconscious into a ragdoll like in Oblivion, they are incapacitated while they recover.
The recovery takes around 10 to 15 seconds, so if you're relying on his power (like I was...) his demise into recovery mode really hurts you as the monsters stop aggroing him and focus on you instead.
