i recently came upon the chapter of Arkovia, where you have to delve into their undercity and such. The lore says that the undead roaming the area cannot be killed by any means? not magic, not weapons, not age, not disease? Unless you’re a player of course, then you can kill them 
the phrase they can’t be killed means that they just crumble and move then respawn somewhere else when you ‘kill’ them. think of it as banishing spirits to other realm for some time before they recover enough power to respawn in the world.