User blog comment:Zathronas/Analysis by an author: What did I miss? part 2/@comment-10380701-20140122000339/@comment-50.153.135.4-20140122021720

I think it's an old wound from a battle that happened before he started teaching. It would kinda explain why he's usually got a cane with him.

Also, I'm a fanfiction writer who dreams of writing my own stuff. I actually used the effect before cause thing you're talking about recently. Gotta love that little trick!