Okay I have a lot of things to say about btvs vampires and scarring that I discussed last night with a friend of mine ( @xaeyrnofnbe ). All/most of this is personal headcanon mostly based on guesses rather than an analysis of canon material.
It’s been stated that vampire immortality is Eternal. They don’t change or grow physically or psychologically after they’ve been turned. I like to think that this idea of Eternity has to be permanently tied to a personal idea of the Self and a specific idea that the vampire has about who they are and what their purpose in life is. I’d imagine that this idea of the self is very important and solid, especially considering vampires’ tendency to self-mythologize.
If something very very significant happens that is so dramatic that it would change that image of the self— then there is some kind of scar. This scar CAN just be purely psychological if there was no physical trauma, but if there WAS physical trauma, then parts of the injuries won’t fully heal and will instead scar over, leaving permanent evidence of the event.
Spike is one of the only vampires that are shown to have permanent scarring AFTER becoming a vampire, so he would be a good example of this possible phenomenon. Spike received the scar on his brow after a fight with the slayer Xin Rong, and after killing her, he redefined his purpose to be hunting down slayers— the event leaving a physical and metaphysical mark to denote the change.
