SetEssential
Syntax:
SetEssential ActorBase bool
Example:
SetEssential MyFavoriteNPC 1 SetEssential MyLeastFavoriteNPC 0
Sets the "essential" flag on an actor (1 or 0). When this flag is set to 1, the NPC/creature will not be killable -- when his health drops to 0 or below, an essential actor will be made temporary "unconscious". This is normally used to prevent the player from accidentally killing an NPC who is required to complete a quest.
Note that it will act as a return statement if ActorBase is a reference variable.
Using on PlayerEdit
Using setEssential on the player will result in camera problems (camera becomes centered on players feet). See UESP: Resurrecting the Player for analysis and alternate approaches.