I mean, walden isn't practicing demonic magic, its just black magic and necromancy. So sure you might get wiffs black magic if he performs it near the group but if he's trying to hide he'll definitely not do it while in the company of a GMO demon and a purifier. The dead language is as old as writing, the book showed up after the first written language. I can't go too much into detail Nariata knows the full details as GM i felt it would interfere the least with him to share, plus he asked. 

I mean I don't care if your suspicious of him, we have a whole week we can't really account for, as the RP started a week after the battle. That area I feel is the GM's territory.