I'd like to thank everyone for all the ideas. I narrowed it down to do main thoughts, both hinging on opening a portal to the Shadowfell.
1 - The bad guy is trying to gain certain Shade benefits (stolen heavily from the 4E Heroes of Shadow book).
2 - He is trying to pause/halt the moon during a solar eclipse to extend the darkenss over the land.
The particulars will have to wait until the next gaming session, as this week was somewhat shortened. They let the Drow escape, although they did manage to kill off the rest of the cultists who were in the lair, and rescued some captives, including their employer, who was escorted to safety right away. They even managed to find the secret compartment that held the gems. The gem merchant told the PCs that the Drow was after a particular gem, but doesn't know the full story. He's not even sure which gem they wanted, as he knew he had several rare and high quality gems, but didn't think any were more than just gems.
The robbery was to cover up the theft of the particular gem, and to help fund further endeavors. The start-up costs for running a cult gets expensive. The gem merchant was considered to be a loose end to be disposed of, and was caught by cultists while he investigated the theft.
Now the drow has the gem, but lacks the safety of his not-so-secret lair, and his minions were decimated. He's suffered a major set-back, but isn't out of the game yet. He just heas to get the necessary resources together before the eclipse in three weeks.
