ConstraintTest

ConstraintTest never reached players. The wiki describes it as a developer-exclusive test aura that cannot be rolled, and confirms it was removed from the game around the Era 5 update, along with a similar unused aura called The Rock. Its odds are technically 1 in 1, the same as Nothing, but since nobody can roll it that figure means little. No creator is listed on the page.

What survives are screenshots. The aura is a bluish-purple cross standing behind the character, with a faint blue portal underneath, and the wiki says the crucifix looks like the one in Undead, even with the same black outlines. That has led to the theory that ConstraintTest was the base from which Undead was built. The page also supposes it was removed because it was only meant for testing. The font is Rajdhani. In the family of removed auras it sits beside Deepwoken and Cruel, which are also developer-side and cannot be earned.