Demona's tiara is a piece of gold decorative jewelry that she's worn upon her brow since, at least, 971 A.D.
In 971, when Angel was the hostage of King Culen, he seized her tiara and considered melting down the gold ad reforging it into a rattle for his infant son, Constantine. When he asked her where she acquired it, she didn't answer. After she was rescued, Mentor returned the tiara to her.
As of 1997, she continues to wear it.