Two Sides to the Coin Perhaps?
Well, after reading back through the thread, I've got to say I think I see the problem. I am however only getting your side of it so this could be a bit off.
You said:
"He introduced them in our 2e campaign because they were immune to turning and there was an undead encounter where our cleric destroyed a bunch of skeletons. He didn't like that so he found undead that the cleric couldn't destroy."
He didn't find them in a 3e book somewhere - he made them sort of to his own specs. And it seems he made them specifically to thwart player abilities.
"And then there is the guy playing the Feral Half-Dragon...ugh. ECL 6 ACL 2. He dies quickly and that's because the DM knows he has a lot fewer hitpoints."
It sounds like you are saying the DM targets this player because he has lower hit points?
You also mention it is a low magic game and the DM refused to let you play Arcane casters and Druids (I could see making some adjustments to the Druid class frankly). I'm all for low magic games - I run them all the time. However, I don't then create challenges that specifically require high level magic to solve without giving the players some option.
It is possible you are having a common problem - it's the "us versus him" scenario where the DM and players are having a competitive, antagonistic sort of game. The DM always wins this of course, but essentially ruins the game for everyone else. A DM's role is definitely to challenge players, but not simply so the DM can "win" the game. The DM challenges players so you guys can create a fun, collaborative story together.
Now, from the DM's side -
Of course, Revanent's don't just run around killing everyone. They kill for a purpose. They hunt down their killer to exact revenge. Maybe you guys were running amok in the campaign and the DM decided to teach you a lesson? I had to blow some dust off my Fiend Folio but, I recall quite a bit about this particular undead. (In 3.5 it would be a template and a rather nasty one at that...)
They are pretty much immune to everything. It is immune to magical and non-magical weapons, turning, holy water, etc and immune to acids, and gases. The only "sure way to kill it" is to reduce it to ashes through fire, which your DM seems have made into a non-option.
So maybe the DM is trying to tell you something and maybe he is deliberately presenting a scenario you can't win through slicing stuff up? You've got to figure out why the thign is after you, what you did wrong and make some sort of restitution or something.
Good news is, it will eventually (after 3-6 months) decompose. I'd suggest a brief vacation to a different plane (assuming the creature was not a former spell caster because it will keep its class abilities and could possibly follow you.)
That all fails, instead of buying a portable hole, put those funds in the hands of a trustworthy cleric outside the party. Commit suicide in sight of the creature, it will go away, and then have the cleric Raise you...or convince the creature you are dead...either way, it should back off.