Essential Characters (Oblivion)

Essential characters refers to non-player characters, which cannot be killed either by the player or enemies. Their Health may be damaged, but they will never reach below zero. Characters are flagged as essential, if they play major roles in quests. Distinct personages, such as Countesses are flagged as essential regardless. Characters may be "unflagged" after certain events occurs, meaning they were once invincible and no longer are.