Brainlessness

Brainlessness is an instadeath caused by mind flayers and master mind flayers. These monsters have brain-sucking tentacles that lower the player's intelligence and cause amnesia. Each time a brain-eating attack is successful, you lose 1 or 2 points of intelligence. If a brain-eating attack is successful when your intelligence is already at its minimum value (which is always 3 in Vanilla NetHack), you die of brainlessness. An amulet of life saving will not save you from death by brainlessness, although you can choose not to die in explore or wizard mode.

Death by brainlessness is only possible due to a monster attack; other sources of intelligence loss (thrones, potions of sickness) cannot cause this instadeath.

Messages

"Your brain is eaten! You feel stupid!"

You lost one point of intelligence due to a brain-eating attack.

"Your brain is eaten! You feel very stupid!"

You lost two points of intelligence due to a brain-eating attack.

"Your brain is eaten! Your last thought fades away."

You were hit by a successful brain-eating attack when your intelligence was already at 3. You die of brainlessness.

"You feel like a scarecrow."

You are in explore or wizard mode, and you chose not to die of brainlessness. Your intelligence is set to 5, minus one or two because of the successful brain-eating attack that would have killed you.

"Unfortunately your brain is still gone."

You died of brainlessness while wearing an amulet of life saving. The amulet "saved" your life, but you immediately died again of brainlessness.

SLASH'EM

In SLASH'EM, there is a new monster that can drain intelligence and thus cause death by brainlessness: the migo queen. Also, if you are a lycanthrope or a vampire, then you will not die of brainlessness until you are hit by a successful brain-eating attack when your base intelligence is equal to its racial minimum, which is 1 for lycanthropes and 0 for vampires. This makes it slightly harder to die of brainlessness. A base intelligence lower than 3 (which is impossible in Vanilla NetHack) will be displayed as "3", so you will not actually see your intelligence go below 3.