Some help with a plot Options

[grahariel]

I've got a plot I'm going to introduce in my next session. Basically, it involves the theft of an artifact from a Wuxing research facility in Seattle. Wuxing has been analyzing and researching this artifact for the last five years. No one knows who stole it, and Wuxing isn't really sure what it does. Description wise, its a circular stone slab that's about the size of a table that sits some five people. It's got strange markings on it that don't really match anything from humanity's historical data, hell, even the Dragons aren't really sure what it does. Okay, now, the idea is that this person has it and is going to use it for something evil (big surprise there). Now, added to Wuxing, Ares and S-K are after it for their own purposes, the Wanabuchi-rengo hope to use it to win a gang war against the other rengos in Neo Tokyo, and a group of Native Americans from Tir Menagierie are looking for it because they're under the impression it might be an artifact they lost years ago (they don't know what Wuxing's thing actually looks like yet). I've got the motivations for the PCs set up already, but what I need help with is deciding what the main villain's plan is.

The villain I've got is a powerful mage whose sworn allegiance to a powerful intelligent Shedim. His plan is to use the slab in a blood magic ritual to open up a gateway into the deeper recesses of the astral plain to allow this powerful Shedim into this world to essentially either devour it or enslave it. (The mage may or may not be possessed by a servant spirit by the Shedim as insurance, I'm still debating on this, but it would explain his powerful abilities, please comment on this aspect at your discretion).

All in all, I want to know if this villanous plot works within the world, and if so, is there anything else I can use to spice it up, or is there something else other than a Shedim I can use. I've had this idea of having the guy put either enthralled bug spirits or blood spirits into the children of key government and corporate officials to lie dormant until a certain time when they'd be used in some kind of simultaneous attack, again want to know if something like this would be possible. And if so, is there a way to keep said spirits from being detected by the PCs when they're possessing a body?

Any feedback is greatly appreciated. I want to make this plot intriguing and plausible for my players.