Arthur shows up purely out of curiousity. He has no idea if he's going to try and take on this specific case, but when there's a decapitated head in a box he has to find out more. You know, for science.

Anyway here he is in Faolan's office, poking his head in and raising his eyebrow with a skeptical look on his face as he says, "I heard a rumour about a head in a box."