Lucifer might be the Father of Lies, but Furtur is the true Demon of Lies. Having been taken in by Lucifer's original lies and unable to unbelieve them, he is now incapable of telling the truth. That is unless he is trapped in a magical circle of some type. If you can capture him, he reverts to the form he wore before he was warped and, then, he loses the ability to lie. Whereas once he brought love to man, now he brings only lust. He is a bringer of storms and tempests.