That was pretty non-obvious. I think you could have handled this little bit of exposition better, if the goblin is so protective over his spell (and people stealing credit for it), why he's showing it to the first random (non-monster) person he came across anyway?
It might have been better if, for example, this was an academic showing that Serrin had gained admittance to. After all, academic papers (and presumably spells) are always peer-assessed before they're released to the public.