Creeperman said:
Congratulations, you've created a meatgrinder. The party has absolutely no chance. Unless they somehow catch wind of the ambush and run, you're most certainly going to rack up a TPK.
I'd recommend taking down a notch or five. An EL 12 encounter should be more than challenging enough, especially with area-effect magic taking out the weaker party members.
Actually, WoTC created the meatgrinder. It's called City of the Spider Queen.
I'm just stuck running it. I can remove the 8 5th level drow fighters, the 7th level drow evoker (that would negate all the lightning bolts, the ice storm and the magic missiles) and the 9th level drow fighter. What would that do? Still unbeatable? The party hadn't noticed these "surplus" ambush drow sneaking up behind them under cover, so they can be safely axed without suspending disbelief.
The party has no chance to catch wind of it until it's sprung, which is where the previous DM left it, at the conclusion of a witty exchange: "surrender"/"over our dead bodies". This is where I'm forced to pick it up in a few days! Bloody hell.
Now, barring an immediate surrender by the party, it's TPK time. The party has only one means of escape; the party wizard can teleport a few of them close to her out IF they win initiative, leaving the rest to die and become undead slaves. Running isn't even an option, as they will be cut down by spells in no time.
An immediate surrender will equate to a TPK, as the drow will attempt to kill their prisoners and turn them into undead slaves at the first opportunity, after some friendly torture for fun, of course.
The only possible allies closeby are a mindflayer merchant (could planeshift most away, but what's in it for him?), the drow archmage who hates the bad guys and wants to control the town himself and has bunches of wizards under his command (but daring to show his hand now over an unknown quantity like the party seems imprudent), or a bunch of paladins and clerics of Lathander who are on the same mission, unbeknownst to the party (who would normally kill them). Ideas?