So I took the base creature as found in the Deities and Demigods and gave it a variety of additional abilities. Primarily, it has the ability to resurrect itself and once alive, it became immune to whatever killed it before. In 2e, you still lost a con point for each resurrection and so it would be permanently dead once you killed it enough times. You just had to kill it a different way. The one caveat was that it could not become immune to dwarven forged weapons (Norse dwarves, not D&D dwarves) which meant that the fighters and rogues were still valuable and became more so as the battle went on.
The campaign was broken up into research and combat sessions where they would find more information about the wolf from other heroes who had fought it and killed it successfully. Hercules had strangled it (It no longer needed to breath after that), Another had burnt it (immune to fire), etc. The wolf was down to about 10 Con but was immune to a laundry list of things. The combat was in the tunnels under Yggdrasil among the great roots of the tree (a separate demiplane) and the beast was chained up by an ethereal collar that tied it to the plane and the base of the tree. The creature's class were sharpness as were the bite attacks (severs random limbs), but anything that the bite cut off was lost forever and could not be regrown. Anybody that the beast managed to swallow whole was also forever lost.
I would like to recreate that kind of game for a 3.5 short campain (maybe a six month to a year game) and I need some help statting it and making it a challenge for anything that people are likely to throw together and this is where I need help.
21st level gestalt heroes. Only one side can be a caster. Everyone must have some martial ability. No epic spell casting.
The ending: Any heroes still alive at the end will inherit an empty god plane and a world in chaotic rebirth. They get to form a new pantheon.
The need: one epic battle to end all battles.