Add the following to the Mind Flayer's stat block.

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Innate Spellcasting (psionics)

3/Day: Ego Whip

Ego Whip

The Mind Flayer magically assaults the ego of one target within 20 feet. That creature must succeed on a DC 14 Charisma saving throw or take 7 (2d6) psychic damage and suffer disadvantage on all ability checks, saving throws and attacks as the target suffers debilitating self-doubt. This effect lasts for 10 minutes or until the creature succeeds at an ability check, saving throw or attack - whichever comes first, this returns the target's self-confidence.  

If the creature makes its saving throw against Ego Whip, it takes half damage and does not suffer any disadvantage.

Ego Whip only works on intelligent humanoids.

(note: i don't know about you, but this might raise the Challenge by 1, as the Mind Flayer is subsequently more likely to stun its opponents.)

Perfect.

Quote from: Monster Manuel;788361Perfect.

Thanks.

Not quite though, i've added that it's only usable against intelligent humanoids.

I know there's history and all, but it might be interesting to allow it to affect any creature with an intelligence over 3. I might do that in my own campaign.

The devs have said that Intelligence should be the saving throw for Psion powers.

Quote from: Strill;788506The devs have said that Intelligence should be the saving throw for Psion powers.

That's nice of them.

For Ego Whip, i've decided that Charisma is the saving throw considering it effects confidence and instills self-doubt etc.

Does the Mind Flayer not have this? I haven't received my MM yet..

Anyways, the description in the OP sounds appropriately Mind-Flayery to me.

Quote from: RPGPundit;789359Does the Mind Flayer not have this? I haven't received my MM yet..

It has innate spellcasting (psionics) at will: detect thoughts, levitate and 1/day: dominate monster and plane shift (self only).

It does have a mind blast with does psychic damage and stuns, but i wanted to give it a bit more flavour with the iconic Ego Whip.

Seems like quite the omission!

That's always been the problem with WotC, they can write mechanics, but their application is...to be charitable...uninspired.

Sweeping Cinder Strike (Badass!) - Pay 2 Chi to cast Burning Hands (WTF!?)