When worn the user gains the ability to cast Finger of Death, however, when the spell is cast the user's mind is filled with horror and unable to sleep, adding one point of exhaustion for every long rest the party takes.  When the player reaches 3 points of exhaustion, the nightmares wear off, and exhaustion points are reduced normally.  The tiara can be removed any time the exhaustion effect is not in play.

With FoD being a 7th level spell, this artifact would be more useful early on, maybe levels 5 - 10, where it can be more of a last resort. I wanted their to be consequences to its use, but nothing permanent, something that made the decision to use it a tough choice, but not one where it never the choice that's made 

Notes: Exhaustion