Acne.
Scarring, disfigurement, hates the breeze on his face but needs to see so he just covers half of it, who knows.
I mean, we do know he has a normalish mouth I guess. In LeBlanc's League Judgement, he was having tea with her, so she's seen him without his mask on.
Visually I do kind of know though. It's meant to make him look like he has a beak. No 'soft expressions' from a regular mouth, so you only get the harsh line look like a bird has.
Actually thought about this when I was writing a fanfic. Personally I think he has some mutation under that scarf, either it's avian like or it's decaying flesh. The crow grabbed a dead corpse with a really messed up leg after all, it only makes sense the body rots from time to time.
My fanfic had draconic...I really should get to typing it up.
Swain is just like PYL, the air is too dry for him so he wears the mask
prubly b/c he has fizz canxcer/
Nah It's just a matter of style.