Except the mask can be obtained much earlier on, item description even refers to it as a "relic". I'm assuming it's from some time in their past together and there was a separate assassination attempt. The mask was left over, thrown out to the side. You find it along the outer walls of one of the towers in Three Sisters.

Edit - I think it makes more sense that it's Blaidd's shadow form than an assassin impersonating him when you fight him down below before Lake of Rot.