Some advice
I am a strong advocate of challenging tactical combats in making an adventure experience rewarding, but let me take a step back for a moment and be the voice of warning.
Its very easy to find yourself in a situation as a GM where you find yourself with players that have build themselves up to the point that nothing is a challenge anymore. In these situations, its really easy to fall into the "me vs. them" type mechanic, which as was written in the DMG or something similar always results in the GM winning.
This encounter I think, if not carefully assembled could be such a thing. After all, most of us reading here are powergamers in our own regards.
With that said, I'd like to offer some tactics that might make the situation challenging:
1. Use deception to your advantage.
Because the party has previously encountered wyverns that have "breathed" and thus will likely assume that, "oh its just the GM's toy again, we know what to expect." This provides an excellent opportunity for deception, be it in the form of an illusion, or as a templated creature suggested above.
Often I find the most memorable combats to be the ones where the PC's honestly have no clue what they are up against, and the things they come to expect from a monster are all but useless information to them. I would recommend using deception in this encounter to your advantage, that being with the wyverns AND with the undead riding them. After all, the first time that wyvern breathes cold, and his buddy breathes fire, and these DEMONIC riders come at them, the party will wonder what the heck is going on.
2. Often the most memorable villain is the one that is played with the intelligence of a PC.
For example, say for some reason the party gets the jump on these bad guys, and just annhiliates three of the four combatants, say with six arrows a round, or some such thing as players are often apt to do. Often if you can get one good round in and then vanish, you've already done the psychological damage. "When will they be back" or "holy crap, what WAS that thing?" are often what you get result, and the PC's don't get another decent night of sleep.
3. Remember that undead NEVER age, and have hundreds of lifetimes to accomplish their nefarious ends.
Most commonly overlooked with undead, is the understanding that the creatures involved NEVER age. What is 10 years of life to them, but a blink of the eye. Eventually the PC's will lose interest, drop their guard, or move on. They can't remain forever vigilant... and as long as the monsters that got away remain out there, they can likely come back and do serious harm... say a PC on guard goes missing and returns as a villain/spawn.
4. Use illusions to your advantage.
While this is mostly covered in point #1, I would highly recommend you consider taking along a wizard who is an expert in playing mind games. Face it, four monsters aren't very intimidating. Four from one side, four from another side, and more on the way is pretty damn scary. Especially when the PC's start wasting spells. Who knows what is going on then?
Anyway, just some thoughts on how to make this a memorable encounter for the party involved. Make it winnable, but difficult, and the players will thank you... that is if you haven't scared the beejesus out of them.
