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Kotaku has a Skyrim preview titled Five More Things I've Learned About Murder, Encumberance and other Stuff, with input from Todd Howard. A snip on immortal NPCs:
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Thanks, MMA!Who You Can Kill and Who You Can't – We may not kill everyone in the massive world of Skyrim, Howard told me, though obviously some have tried such slaughters in previous games made or overseen by his studio. "Central" characters in Skyrim will not die. If defeated in combat, these people will drop to their knees. Rare "Protected" characters can be killed by you, but not by non-player characters who do have the ability to murder their fellow Skyrim dwellers during the kind of unplanned, dynamic chaos Bethesda players enjoy igniting. Those Skyrimians who are neither Central nor Protected may die by your or others' hands—to be clear, those "others' hands" are computer-controlled, as this is as single-player a game as it gets.
The man to whom I aggressively delivered a few arrows, I'm sad to report, had friends who proved to me that my character was neither Central nor Protected. Hey, but at least I'm the hero. I can respawn.
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