Eat Your Fill
It was something so obvious and yet, Evian had not realized it; Innate abilities did not cost him Eldritch energy.
Eldritch energy was for spells or martial abilities but Innate abilities were something more. They were abilities fundamental to Evian being a Wraith.
Eldritch Authority! was a force of will. Evian just had to 'will' every bit of Eldritch energy within the body of his victim to obey his command and it had been so instinctive that he had not stopped to really think about it.
'Visions of the Eldritch' was a force of the mind. A mental attack of insane proportions that could attack even beings that were not made up of Eldritch energy. A grace that was not afforded to Eldritch Authority which was likely why its blowback and cost were so great.