Why Un-damageable? If the PCs attack them do they get attacked by level 50 guards?
Default AC is 10 without armor or other gear, plus any Dex modifier, which I assume most commoners have 10 (+0).
I'm having trouble following the idea of immortal NPCs, not sure why that would be valuable.
What is your own approach to representing commoner NPCs in your game?
What is your own approach to representing commoner NPCs in your game?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.