How Would You Mexican Stand-Off?

[Tevesh]

I've been running a game for over a year now and I'm stumped with an encounter I have in my head. I would really like it to go this way which is why I created an account to get the wisdom of the forums. GiaP has been a life saver for me. Many of my questions were answered by a mere google so I hold the answers I get in high esteem.

I am heavy into crunch because I don't want to just make up DCs that are impossible to beat or the like. I'm playing with 3.5 Monte Cook's Experimental Might so the PCs can handle a lot being thrown at them. They're very powerful.

So, this is the situation:
Party has been negotiating with Devils and Demons, playing off each other for the Greater Good, yadda yadda yadda.

I want a coincidence of Demons and Devils making a move on the PCs at the same time. This would sort of be a Mexican stand-off situation where both factions realize that the PCs have been played them.

Any special rules for the set up?

How should I CR the encounter? If the 3 PCs are cornered by 3 Demons and 3 Devils, that makes them pretty wimpy individually. I sort of picture the 'big bad' attacking the PCs while the flunkies fight each other; however, that might be a lot of book keeping involved.

Or just do 1 Devil and 1 Demon that work together but attack the PCs and the flunkies are just literally wallpaper? Not the first time I've done something like this but I feel that its somehow cheating.

PCs would be level 18. If you can suggest some interesting Devils/Demons, that'd be nice. The PCs would've gone through Graz'zt on the Demonic end, Mephistopheles and possibly Glasya on the Infernal. If there is a particular monster or methodology they would use or Template, that'd be flavourful.

I could go for the quadruple Mexican Stand-Off of PCs vs. Grazz'zt Agents vs. Mephistopheles Agents vs. Glasya Agents, but that's sounding a bit ambitious.

Bonus Points:
I've also been trying to incorporate a Deadborn Vulture as a Mount for a Caster but I have no idea how to scale it up according to CR. Sure, the creature given in MM5 is CR 8 but if I increase its HD its size increases and then how should I make its Skeleton? Or just increase its size and all it a day?

The issue is that the PCs will be 17 or 18 when I'll want to use the Deadborn Vulture Mount, I don't want the Zombie version to be a complete gimp.

I think its a neat mechanic, a different version of "Bloodied".

[John Longarrow]

Thematically,

I'd have both sets of outsiders behind some kind of barrier that protects them from the other. Both are waiting on the PCs to show up (they know they are enroute) and each side thinks the PCs are there to help.

Let the fireworks start when both the Devil and Demon in charge call upon the PCs to assist them. Neither will be surprised the PCs are helping the other (after all, they ARE lower planar beings) but this will force the PCs to either side with one group OR fight both.

I'd have the Demons and Devils fight each other more than the PCs. Toss in the fun they have with teleport and you should be able to keep the PCs kinda sidelined. After this fight both groups will start making the PCs lives very difficult until they figure out how to solve their new issue.

[Lachdan]

I would make both groups a CR 18 encounter with a good mix of individual creatures.

Then grab someone and give him the demons or devils and an objective (there should be something other than just murder everyone) to play and take for yourself the remaining faction.