Taboo Move.
He went through her history and found her identity. Unfortunately, he didn't know at the time that she had stolen someone else's identity. She was masquerading as someone else, and Mother High Heaven didn't notice. He knows now that it is because she can change her spiritual flunctuations into that of anyone else's.
But what further drew his attention back then was how she waltzed through the trial. Nothing could harm her. And even though she didn't attack anyone at all, she still managed to make it to the final trial.
Of course, now he knows that it is not that she didn't want to attack but that she can't attack. Somehow, she is completely invulnerable to attacks, as they will become intangible and disappear if they come near her.
That is the only reason he can remain calm with such a person around him. Even then, he decided that he had to start the era of conquest as soon as possible.