[quote=@Lugubrious]
Karn's immortal?
[/quote]

Not really. He's just being kept alive by some magic. Eventually, that magic will run out. It's like using a magic spell constantly. It's like Loki having to stay in the human world. His power is strong that kept him going for a lot longer than most Celestial Spirits would but that power eventually ran out. Same applies.

In this case, it's like his heart is getting shocked awake again.

Just prolonging fate.

But he'll still die die if he gets decapitated or such. It's just if his sickness kills him xD

It will be explained Better in the 2nd personal quest