The Lying Iris is the Iris possessed by Asahi Yuki.
The ability to "see through lies".
The ability of this Iris allows the user to see when people are lying, by seeing the liar grow a devil's tail out of their behind. This includes the owner of the iris so if they lie, they would see themselves grow a devil tail.
It should be noted that if the person believes that what they are saying is the truth, they would not grow a tail.