A commander with (X) amount of Insanity (formatted as "Insane (X)") will have an X% chance before the start of each turn to refuse accepting commands from the player this turn and perform some random action instead. Their little square will be colored red instead of the usual black. They may:

• Patrol
• Site-Search (or at least attempt to)
• Enter a local Site (or at least attempt to)
• Research (or at least attempt to)
• Preach (or at least attempt to)
• Call for their God (or at least attempt to)
• Hide
• Hunt for Blood Slaves
• Claim the local Throne (or at least attempt to)
• Become a Prophet somehow, even if you already have one
• Waste the month searching for "signs" (no effect)
• Waste the month agonizing over a star that's not a star (no effect)
• Waste the month absorbed in the Dreams of R'lyeh (no effect)

They have a 50% chance to do one of the stupid useless things, and a more-than-50% chance to be useless overall (unless they're Mages in a special spot).

You generally don't want to rely on Insane commanders, and you certainly don't want to render your Commanders Insane if you don't have to. The following give Insanity:

• Holding the Dimensional Rod (+1 per month)
• Lingering in Late Age R'lyeh's tainted Dominion (+1 per month)
• Using the Void Gate ((65/Void Sanity)% chance, +2d(65/Void Sanity) DRN)
• Entering The Void (50-100% chance (divided by Void Sanity) of +1d6 DRN per month)
• Chugging Mercury to de-age yourself with Internal Alchemy (typically +5; less if you're at or above 50, but never zero)

A Commander with a Shattered Soul (X) has a more-dangerous form of Insanity, typically brought about by being in Hell for too long. They have an X% chance of refusing input and doing something stupid, like Insane Commanders, but their list of actions is generally far more destructive. They might:

• Pillage
• Destroy a local Temple
• Research (or at least attempt to)
• Preach (or at least attempt to)
• Call for their God (or at least attempt to)
• Site-Search (or at least attempt to)
• Raise Ghouls from the local populace
• Do something scary but ineffectual (30% chance)

Nothing increases your Soul-Shattered-ness, thank gosh, at least within the scope of the game.

Void Sanity (X) divides your Commander's chance of going insane for a month by X; for example, a guy with the maximum amount of Insanity (100) and the maximum amount of Void Sanity (20) has only a 5% chance of losing it each month. Void Sanity reduces insanity gain from certain sources, as listed above.