When Raze's blade met the resistance of the unseen mage's defense, he knew instantly, based on the sheer density of the mana, that it could only be one individual.
Even before that, from the surgical precision of the attacks, it seemed like there was only one person involved in this massacre. Would Idore really send out a single mage to take on dozens of Guild leaders and the Dark Magus himself? No. Unless that person was someone Idore trusted above all others.
And once Raze had deduced that identity, he threw himself into the fray with a ferocity that bordered on suicidal. There was only one reason for this level of desperation.
