In Book 13, SOUL SHADOW, you get a glimpse of the nightmare that inhabits L'Ni's mind. This was hinted in Book 8, RACING THE ORAST BELT.
It is the evil that Nya and her friends are fighting, the creature that seeks to destroy the universe while promising the Feri a new creation where they rule. This creature manipulates them to serve it relentlessly with lies. Its hiss is the source of its name, Issa.
It wasn't too difficult to get this image, but it's pretty generic. I did have an idea of an amorphous black smoky shape with red eyes like I described as coiling around Nya and L'Ni in Book 13, but the face was not specific.
Can you imagine what this nightmare would do if it escaped into our universe?