So - the "big bad" is an illithid. They are into all the nasty Mind Flayer stuff. They are (nominally) based in the Underdark.

Now they need a top henchman. Some top-shelf muscle who stands behind the mind flayer and looks intimidating. Someone to throw at the PCs who they would engage on a physical level. Someone who - presumably - the Illithid wouldn't be tempted to turn into a snack while famished.

What race should i make this henchman, given that they'd be spending most of their time in the Underdark?

(FYI - I've ruled out Drow, as it wouldn't make sense with the story.)

Thanks.

You haven't mentioned a level but how about this:

If in an area with large open spaces then they are accompanied by a Fomorian Giant.

If in smaller caves and tunnels then they have a Zombie Beholder except, should the illithid be slain or planeshift away, the "zombification" ends and the beholder reverts to a normal beholder.

If the Illithid is expecting trouble then they have a unit of Quaggoths lead by a Quaggoth Thonot, the number of Quaggoth in the unit is equal to the number of opponents they face.

For infiltration purposes they have bred an adult Oblex.

For hunting and scouting purposes they have several Intellect Devourers.

Intelligence devours or human thralls with intellectual devours inside thede heads .

Quote from NewbDragonslayer >>

So - the "big bad" is an illithid. They are into all the nasty Mind Flayer stuff. They are (nominally) based in the Underdark.

Now they need a top henchman. Some top-shelf muscle who stands behind the mind flayer and looks intimidating. Someone to throw at the PCs who they would engage on a physical level. Someone who - presumably - the Illithid wouldn't be tempted to turn into a snack while famished.

 

 I have no specific suggestion. The requirement that they not be likely to eat them doesn't mean much, since mind flayers are capable of controlling themselves.

But the fun option is to have whoever it is obviously being controlled by the mind flayer, so the pcs may be reluctant to kill them.

Well, Grimlocks are also a by product of Illithids and their terrible rein. You could also use Grells, Illithids would definitely be powerful enough to dominate Grells and it fits with the Brain theme. Nothing looks more intimidating than a tentacled human control flying brain birds with even more tentacles.

Umber Hulks are popular for use as "muscle" by mind flayers.

Construct.  Lean into the sci fi aspect of mind flayers and give them a robot.

Mind Flayers are traditionally shown having a distaste for constructs and non-living stuff in general.  They prefer live organisms and organic bio-tech.

Split the difference?  Giant flesh golem is biotech and a construct.

Don't mind flayers also use dueregar as minions? 

Quote from CharlesThePlant >>

Don't mind flayers also use dueregar as minions? 

 Mind Flayers will use pretty much anything they can successfully enslave as a minion.

Quote from Rob76 >>

You haven't mentioned a level but how about this:

If in an area with large open spaces then they are accompanied by a Fomorian Giant.

If in smaller caves and tunnels then they have a Zombie Beholder except, should the illithid be slain or planeshift away, the "zombification" ends and the beholder reverts to a normal beholder.

If the Illithid is expecting trouble then they have a unit of Quaggoths lead by a Quaggoth Thonot, the number of Quaggoth in the unit is equal to the number of opponents they face.

For infiltration purposes they have bred an adult Oblex.

For hunting and scouting purposes they have several Intellect Devourers.

This is all very helpful!

The level is unimportant, as this character will distinct. I can always take an existing creature and crank it up for an appropriate CR.

For this purpose.... the Fomorian Giant sounds perfect.

Thank you!