The Reason
On the inside though, he wanted to say that his Blood Blade was a million times more powerful than any of their skills but managed to keep it in. Still it was a fact that these individuals were quite powerful among their peers, quite exceptional if one were to judge them based on their attributes.
Despite how powerful they were, Zavier could have beaten all three of them even if he was on his own. He had that much confidence in his strength at the very least. Fighting against an S rank mage and facing two monsters beyond S rank would give you at least that much.
But the condition for that was Blood Blade. Zavier would have to use the cursed technique if he wanted to deal with all of them at the same time.