I am debating on Boeruine's sword named Kingsbane to have the quality of a tighmaevril weapon. In truth I think it balances the forces of opposition between Avan and Boeruine. Being 12th level Boeruine can become a grand master with his bastard sword, thus making direct combat with him deadly and would explain the encounter he had with Rhoubhe from the Ruins of Empire book.

This does give him an edge over Avan and many DMs would think it is too much so I am wondering what would you do as a DM to counter this.

I am not say that his sword is tighmaevril just that its enchantment gives it the effects of this type of weapon.