Not all followers in The Elder Scrolls 5: Skyrim are created equal, and there is no better proof of that fact than the existence of essential followers. These NPCs are internally marked as significant characters, usually due to their involvement in important quests during the game's primary storylines. As a result, they are impossible to kill by any means; bringing their health down to zero will do little but temporarily incapacitate them. While other followers can perish (rather easily) in their service to the Dragonborn, Skyrim's essential followers will stick by the player's side through the toughest of battles, and that makes them some of the best company to keep around.

https://screenrant.com/skyrim-followers-that-cant-die-essential-npcs-companions/