There is a difference with shiva.
Ysayle used the crystals gathered for Ravana the first time, and didn't maintain her primal form long enough to truly affect the world. The second time at Azys Lla, she used her own crystal of light, forfeiting her blessing of light to turn into Shiva for the last time.
On the contrary, Thordan was aiming to wander around sucking aether dry in the eventuality he couldn't find ascians to consume.