Source: Volo's Guide to Monsters

Weapon (any sword), rare (requires attunement by a specific individual)

Mind flayers can turn any nonmagical sword into a mind blade. Only one creature can attune to it: either a specific mind flayer or one of its thralls. In the hands of any other creature, the mind blade functions as a normal sword of its kind. In the hands of its intended wielder, the mind blade is a magic weapon that deals an extra 2d6 psychic damage to any target it hits.