So if I understand you correctly, National Acrobat, you have what essentially boils down to a 10 on 10 fight. Yikes! As a DM, my head would be spinning at the logistics of such high level combatants slugging it out.
The question you have to ask yourself is, "Do you want this to be the end of the campaign?" I'm disappointed to see that so many people have replied under the opinion that you should wipe out the PC's and teach them a lesson. Damn, what a painful lesson for you! 2 years worth of campaigning is a feat that most DM's can't pull off and one that most players would kill for.
I don't understand such truly suicidal behavior in high level, long-term characters. How did they get to level 12 if they exhibited this kind of behavior? Did you pull your punches? Did you not sufficiently challenge them? Did they just always get lucky, via DM fiat? Who knows.
I'm inclined to tell you that this should be an epic battle, but not a game stopper. It may well mean the death of a few PC's, but when you have 10 of them, maybe you can spare to shed a little bit of their blood. I would not advocate that you simply watch the dice fall. As a DM, I don't know how many times I've tweaked dice rolls for a more dramatic result. No matter what anyone else says, there is no drama in having a PC die just because of bad dice rolls. Do what you have to do in order to give the PC's a fighting chance. Just don't make it obvious.
I read your post and envisioned this meeting of the powers, like Lee and Grant at Appamattox. Only Lee knows that Grant is going to double cross him and is ready for it. It's going to be a titanic and bloody clash! Possibly a last stand. Play it out as such. Don't turn it into a route or a slaughter. Make it a pitched battle, even if that means you have to fudge the dice. It's been my experience that players like having their asses handed to them, only to persevere by the smallest margin. And sometimes, if you're real good, you can have them saying to each other, "Man, the dice were brutal tonight, but that one lucky 20 I got saved the day." That's if they're thinking about it from a mechanics perspective. I always try to keep that in the background, though. It makes my "cheating" less obvious. But you can still fool them.
That said, let the PC's "win." Have the henchmen put up a good fight, but ultimately fall before the PC's might. Have the three ranking drow soundly whip their ass, but be sure to have at least one of them die. Kill off a few PC's; those who are being especially fool-hardy die unspectacular deaths, but those who are demonstrating true heroics get the glorious death. Don't plan the deaths ahead of time, of course, but keep in mind that this might be the outcome. Maim the PC's. Leave them bruised and broken while their hated foes flee the battle, only to return with a newly amassed army. Make them pay for their foolishness with more than just their own lives. Make it the lives of those people they've been trying to protect all this time. The drow will come back with a vengeful fury and it will be felt by the people of the land. The PC's name might even be cursed by those who suffer the attrocities committed by the resurrected drow forces.
In other words, you're presented with the truly difficult task of simultaneously whipping the PC's good while letting them pull out at least a draw, having it look good, feel realistic, teach them a lesson, and keep your campaign from ruin. Not easy, but rewarding when you successfully pull it off. It'll be a fight that your players will talk about for the rest of your gaming careers. And hopefully, they'll look back at it in retrospect and say, "Man, that was one hell of a fight. What in the world were we thinking?"
Gaius