The Legs of Lolth
The characters in Mysteries are high-level adventurers, but have relatively little fame for their exploits because of the isolated part of the world they generally work in. All of the characters need to have a reason for helping out the peoples of the jungle (or at least most of them). They may be frequent adventuring partners, or peers who only team up occasionally, but they do at least know who the others are and have at least a reasonably functional working relationship.
These characters are basically meant to be used once, but if we do several of these adventures you’re welcome to keep one going or swap them with each other instead of coming up with all new PCs. I’m not yet decided on how I want to handle possible leveling up, but that won’t be an issue for a few adventures.
PCs will be ECL 15, pretty much however you want to get them there. Everyone will also get an extra 10,000 XP to have spent on crafting items, LA buyback (following the Unearthed Arcana rules), keeping in reserve for high-level spells, or anything else that might cost XP. Alternately, you can swap out for 2 Prestige points for every 1000 XP you don’t want to use or keep in reserve. These can only be spent on permanent abilities.
These characters will use the same rules as The Legs of Lolth for weapon proficiency, but I think we’ll forgo prestige points, supernatural bonus feats, and first-level NPC classes. Supernatural nontransparency will also be in effect.
See the link for equipping your characters.
If you’re inclined to develop your character, check out The Serpent Jungle for ideas and considerations.
For more specific details on the process, see “Creating PCs Above First Level” in Chapter 6 of the DMG.