I Promised to Use this as a Last Resort, Like NOW

Nia gave Zeydaan/Isabella a copy of her core crystal several years into their friendship. The raw ether that comes from it, as well as life energy from the gormotti blade's healing energy make for a battery of energy for them to use as reserves for her transformation energy.
But there is an added bonus. If she grasps it tight, she can absorb all of that energy at once, not only granting her Nia's form, but breaking the usual limits of her transformations by safely granting them access to Nia's powers too.
Here we have an example of that, Isabella pushed to the wall on a Hawkmoths mission, no more energy to use to transform, she pull out the crystal as a last resort.