Psionic Blast is an innate Psionic Ability available to the Mind Flayer race. The ability is initiated via script by the AI. The effect is directed at a target in visible range of the source creature. The blast will require those creatures in the area of effect radius of the target to make a Saving throw vs. spell or suffer a Stun effect that lasts one turn. The effect is non-magical and can't be dispelled.
Players who have transformed into a Mind Flayer via the 9th-Level Arcane Spell Shapechange have access to this ability via the innate abilities' menu. Transforming into another form, then back to Mind Flayer will reset this ability, allowing it to be cast again.
The Psionic Blast will bypass Magic resistance and cannot be blocked by spell protections such as Globe of Invulnerability or Spell Trap. It is party-friendly if cast by a Shapechanged party member.
A more potent variety of this innate ability is often used by the more powerful illithids, such as the Ulitharid - assigned as SPIN959.spl. This Psionic Blast variety has a -4 saving throw penalty.
Note[]
Many Githyanki (Race) opponents have scripted use of a variant of Psionic Blast (which is a different spell), and during combat text displayed on the player's combat log will indicate when it is used.
Internally, in the creature's script the spell is referred to as either Psionic Mind Blast or just Psionic Blast. See the link to read about the differences between the illithid and githyanki versions.