When Izroth first arrived within range of Ourami just moments ago, he used the Crystallization effect attached to his Apparel of Insatiable Transcendence to seal the pillar's movements.
Izroth understood that the effects of Crystallization would not hold Ourami for long; however, his purpose was to create a window of opportunity to use his Boundless Nullifying Expanse.
'His form has not changed even when under the influence of the Boundless Nullifying Expanse. That being said, he does not seem entirely unaffected by it.'
Izroth had been observing Ourami carefully and noticed that the abilities he used appeared to be contained within certain parts of his body.
This made Izroth think back to the Mandates he came across previously.
While the Mandates were not an actual part of Ourami, it was likely that each was gifted a portion of one of the pillar's abilities.