The World’s Largest Dungeon
Some pointers on character generation so that no one goes in feeling that they’ve created a character that won’t work well.
All Player’s Handbooks, All “Complete” Series Books, All “Races of …” Series Books, The Book of Exalted Deeds and The Book of Vile Darkness
The further from Neutral you are the bigger the risks and rewards there will be for your character, perhaps even more so in this module than normal. To eliminate inner-party conflict all characters should be within one predetermined step of each other on the good/evil axis.
Wizards and Druids get hosed in a special way due to an inability to find plentiful spells and and re-commune with nature and find replacement animal companions.
Any CR 1 race from the books above is fine. Just keep in mind this is a major spelunking mission and some races are far better equipped for that task than others.
Very Useful: Balance, Concentration, Decipher Script, Disable Device, Heal, Hide Intimidate Jump, Listen, Move Silently, Open Lock, Search, Speak Language, Spellcraft, Spot, Survival, Swim (late game, maybe), Tumble, Use Rope
Useful: Craft (Smithing), Handle Animal, Sense Motive, Use Magical Device
Less Than Useful: Disguise, Forgery, Gather Information, Perform, Profession, Ride, Sleight of Hand
Pretty much any established God will be fine, players are encouraged to seek the graces of at least one God even if they are not clerics, however it is not required.
Other Notes from the DM
Magic from the school of Conjuration is slightly weaker in the dungeon as the dungeon is treated as a plane-locked area. Spells that make use of planar travel and teleportation are likewise effected.
Go for a 4 person party. Have backups in mind. Don’t stray too far from the classic 4-man party of fighter, rogue, cleric & wizard – it works for a reason.