Source: Volo's Guide to Monsters

Weapon (whip), rare (requires attunement by a mind flayer)

In the hands of a creature other than a mind flayer, a mind lash functions as a normal whip. In the hands of an illithid, this magic weapon strips away a creature's will to survive as it also strips away flesh, dealing an extra 2d4 psychic damage to any target it hits. Any creature that takes psychic damage from the mind lash must also succeed on a DC 15 Wisdom saving throw or have disadvantage on Intelligence, Wisdom, and Charisma saving throws for 1 minute. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.