The Mastered Void Gate.
It was the first of three steps to the special powers of a Voided Deathform.
A user of Voided Death interpreted the void in their own way to establish what kind of power they could be granted through the Void Gate. Skullius had seen one interpretation of the void by Aurolio, where he manifested strength in many different forms – clones, impressive defense, etc.
Once a user was inspired and decided the kind of power they desired, a Bare Guise would be a formed, a mask that noted their achievement and qualified them as a non-standard Voided Deathform.
It was clear to Skullius already that both Elita and Uyuniya were much better Voided Death users than Aurolio. For Aurolio couldn't have contended against him in the least, not with the power he had now.
Because of this, even though the two ladies were trapped in his Dousing Sanctum, he didn't consider them defeated yet. Instead, he chose to take them a lot more seriously.