In the continent of [insert name here] the various humanoid nations have been united in an age of prosperity after the last Great War. With the rapid increase of population, resource demand and the lack of internal enemies, both governments and private organizations are launching forays into the wilderness; clearing forests, exploring mountains for mineral reserves, draining swamps, building outposts... and hunting monsters.
Unlike mere animals though, many monsters are intelligent - and most of them have some sort of civilization. Naturally, the humanoid nations don't really care. Also naturally, monsters are somewhat opposed to their being hunted down en masse and becoming extinct in the face of humanoid progress. And while humanoids have large groups, wealth, technology and centralized organizations, monsters can wield tremendous individual power - and some of them are smart enough and like their unnaturally long lives enough to cooperate in the face of annihilation...
Each of you plays one of the strongest, most influential monsters in a fairly large region; a powerful dragon, a terrible fiend, an ancient undead creature, a giant king/queen, a primal elemental, an eldritch horror and so on. Living in the first area the humanoids try to explore and clear en masse, the first order of business is survival. If you succeed, you can then cooperate to halt and push back the humanoid intrusion. And if you succeed in that... an eyestalk for an eye and a tooth for a fang; invade the humanoid kingdom in turn.
Rules so far:
1) Make a CR 23 monster using monster advancement rules. (look them up in the SRD)
2) You have 370k gp for items, unless your monster started with triple standard treasure (ie dragon) in which case you get 600k. Keep in mind that monsters may not have access to markets later in game and that some monsters also cannot use some item types.
3) If you want class abilities, you can take class levels. A class is associated if you have a racial or HD ability that directly boosts it such as innate spellcasting boosting sorceror levels or full BAB boosting a full-BAB class. Associated classes cost 1 CR per level. Unassociated cost 1/2 CR per level.
4) Remember that monsters DO NOT use epic saves and epic BAB.
5) If you get Epic Spellcasting, the maximum unmitigated DC for any spell is three times the Spellcraft ranks of the caster(s). This cuts down on ridiculous rituals.
6) HPs are average. Skill points from intelligence increases are retroactive, just like all bonuses from other ability increases.
7) Infinite loops and similar broken stuff are not allowed. Creatures from the ELH and other epic sources are not available for play. (dragons are an exception to this) and the tauric template is also not allowed.