In classic versions, rage doesn't grant immunity to stun from mind flayer's Psionic Blast [Confirmation Needed].
Once the effect duration ends, the Barbarian loses all additional hit points gained due to the additional Constitution, to a minimum of 1 HP, causing them to fall unconscious.
|“||The character is enraged for 5 rounds, which grants him/her a +4 bonus to Strength and Constitution, a -2 penalty to Armor Class, and a +2 bonus to Saving Throws vs. Spell, as well as immunity to all charm, hold, fear, maze, stun, sleep, confusion, and level drain spells.||”|
- Barbarian Rage is named Rage, which is the same name as Enrage in class description, both of them can be seen in character generation as well as in record menu during gameplay