ToME: the Tales of Maj'Eyal

I've seen 31 birth unlockables in wiki, but there's 32 total options, which one is missing?

wiki link http://te4.org/wiki/unlockables

Insane game mode?

My unlocks
Birth equipment: Transmogrification Chest
Campaign: Infinite Dungeon
Campaign: The Arena
Class: Afflicted / Cursed
Class: Afflicted / Doomed
Class: Celestial / Anorithil
Class: Celestial / Sun Paladin
Class: Chronomancer / Paradox Mage
Class: Chronomancer / Temporal Warden
Class: Corrupter / Corruptor
Class: Corrupter / Reaver
Class: Mage / Archmage
Class: Mage / Necromancer
Class: Psionic / Mindslayer
Class: Rogue / Marauder
Class: Warrior / Brawler
Class: Wilder / Summoner
Class: Wilder / Wyrmic
Race: Undead / Ghoul
Race: Undead / Skeleton
Race: Yeek / Yeek
Talents: Cunning / Poisons
Talents: Spell / Ice
Talents: Spell / Stone
Talents: Spell / Storm
Talents: Spell / Wildfire

I've seen a stone warden in class list. And I'm working on Adventurer,Solipsist,Zigur Betrayal,Redhead cosmetic. Birth shows 26/32

Is insane mode = adventurer? Or how to get it? What's the one left?

The one left is insane mode, which is unlocked by beating nightmare mode.

Insane mode is simply a difficulty level that's even higher than Nightmare. Which sounds insane so I guess it's well named.

And why stone warden is selectable but not count as an gained unlock?

Because you don't unlock it. It's given as an addon to donors. You either are a donor and have it, or are not and don't. It is not unlockable at this time.

I am a donor, but when logged in, it shows 26/32. And 26/31 for offline. So there's another online unlock? Is Items Vault count as unlocks too?

It's probably the redheads. That's donor-only and you need to unlock it in-game.