For monsters not from nature endowed with a mind, and who are therefore not perceivable through telepathy, see Telepathy#Mindless
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 base intelligence is already at its minimum value (which is always 3 in Vanilla NetHack), you die of brainlessness.
A ring of sustain ability saves you from the intelligence drop, and thus can be used to prevent death by brainlessness, provided you put it on when your base intelligence is still higher than your racial minimum. However, if you put on the ring when your base intelligence is already at its racial minimum, then you will die of brainlessness the first time the mind flayer hits you with a successful brain-eating attack. Like the dunce cap, the ring does not save you from forgetting levels and items.
- 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 base 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 base 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.
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.