How should I use this mask my players found? (Story help)

[MonkeySage]

In their first adventure, my players found a mask and a letter written in Calim****e. According to the letter, the mask was to be delivered to the writer's contact in Tethyr.

My player took the mask and the letter to the local sherif. She was less than thrilled to see it, and has asked the barkeep to send word to her Harper contact.
That Harper, when he finally arrives, will know enough about the mask that he too will be worried about it. He'll extend an offer of work to the party.
Basically:
The mask is bad news for the people of Tethyr. I never assigned it a real purpose, though. I don't actually know what the mask does, beyond than that it has some sort of link to the Illithids. The person this mask was sent to is an illithid, and he wants to stoke the flames of conflict between Tethyr and Calimshan, somehow using the mask.

Before I get started though, I'd like to have a clearer idea of what this mask can do... or if I should keep it on the shelf until later. My players aren't going anywhere yet- they're busy dealing with an unnatural drought.

[Jay R]

One possibility is to leave it open, and then build it out of the players' most evil guesses. You let them do the hard work for you, and it winds up more terrifying to them than anything you'd have come up with.

[Koo Rehtorb]

The mask has some useful magical powers to a ruler. +Charisma, Persistent Detect Thoughts, etc.

But it's also cursed to slowly erode the wearer's mind and make them suspicious and distrustful of everything and everyone.

[Lord Torath]

Buffy, season 3, episode 2 - Dead-Man's Party.

Summary: It animates near-by dead as zombies. If one of the zombies puts on the mask, it is transformed into an avatar of a zombie-demon/god/thingy.

[MonkeySage]

"Do you like my mask? Isn't it pretty? It raises the dead!" XD

Not a bad idea, I think I can work with that.

[Bulhakov]

How about everyone near the mask sporadically gets vague prophetic thoughts or visions about bad things in the near future (but either too generic or too specific to be useful, e.g. someone sees a floating eyeball in their beer, next day a random shmuck loses an eye in a barfight). Should be enough to make everyone feel uneasy and worried that the mask may be causing these bad things.

Wearing the mask gives very clear visions of future disasters/invasions/murders, but only just after the wearer kills someone.