Vader suffers from Acute Neumonoultramicroscopicsilliconvolcaniconiosis (yes, it's and actual illness).
It's a disease where lung tissue is damage or scorched away from inhilation of microscopic volcanic ash and/or molten rock. I'm guessing he did most of the damage when he was gasping for air heavily while clawing his way back up the black sand.
__________________ Land of the free, home of the brave...
Do you think we will ever be saved?
In this land of dreams find myself sober...
Wonder when will it'll all be over...
Living in a void when the void grows colder...
Wonder when it'll all be over?
Will you be laughing when it's over?
I noticed that scar and was wondering about it too. The Large gash behind his head. I would guess that it may have occured during the twenty some years before EP5,6. But there is no one who can really stand up to someone like vader. Maybe the Emperor got pissed off at him one day for showing to much forgivenesss for his admirals or someone, then swiped him with his lightsaber for punishment.
all it means is that, that part of his head was on fire longer than the rest of it.
__________________ Land of the free, home of the brave...
Do you think we will ever be saved?
In this land of dreams find myself sober...
Wonder when will it'll all be over...
Living in a void when the void grows colder...
Wonder when it'll all be over?
Will you be laughing when it's over?