What good timing. I literally have a player who wants to play a Northman in Yrth who has a "willful weapon" like this. He wants it to grow with him, so I think the best choice is to build the weapon as an Ally. I will have to check out the rules in Thaumatology.
My question would be: how bad would it be to just use the Willful Weapons rules from
here? (which I assume is what the OP is referring to)