Finding a one-armed man shouldn't be so hard for Darkling et al - but I don't know what's likely to happen now Grain/Zack/whatever has run into their enemies.
My vote goes for: Grain does the stupidest thing he could possibly imagine.
Probably... pretend to be a dark power like an idiot. It's not like he has a left hand to check but also... asking him to cast magic but... he does have 'This light power git, Alistair, chopped my arm off.' Surely they'd remember the guy they were attempting to torment, and Grain can become an ally via common enemy.