Need a army for a Beholder.

[Throne12]

So i have a city that sitting a a big vain of crystals that Amplify magic and psyonic powers. The population of this city is made up of gith and a lower class homebrew race of rock like humanoids. I want a BBEG come in with a army to lay seage to the city. While there a conflict mapping between two factions. The Githyanki and the Githzerai.

My players are 7 level, blood cleric, adj wizard, oathbreaker paladin, a gunsmith Artificer, (and blood hunter, beast ranger but they moved and might not to able to Skype in).

I was thinking maybe so nothic's but that it. The beholder I was going to reskin it as a round crystal with different color smaller crystals rolling around it as it eye stocks.

[jjadned]

You could always reskin like lesser beholders as crystal monsters, or just reskin rock elementals

[Joe the Rat]

Nothics just feel right for beholder minions - I went there as well. If you describe the spiny growths across the body as crystalline, that'll fit.

Some modifications:
The secret-extracting gaze can be mod/amplified to affect psionics - each successful gaze stealing one of the psionic abilities of the various Gith - this makes them a bigger issue for the city, but no greater threat to the party - so no CR adjustment.

Creepy rotting and secret-stealing gaze, and a potential AC adjustment can turn anything into a Nothanalog. I used this to have some minotaurs transformed into Nothataurs - spiny cyclopean minotaurs with the ability to see the places you've been (Their maze-mastery translating into seeing the paths you've traveled).

Nothics are the Lair: The Ioun Monstrosity (your crystal beholder) can see through any of it's Nothics. You could have the eye-beam Lair Action manifest through them, but only if the boss is within a certain range

Spellcasting Nothics: Heh heh heh.

Other ideas
You might try adapting Wights as an officer-class, swapping undead for abomination, and having their drain convert the dead into Psi-zombies (Zombies powered by a big-old crystal stuck in their heads) or a nothic-creation process.

If you have Volo's, there's a few beholder-variants that might be useful as commanders. Also take a look at Deepspawn for something a little more stealthy.
If you don't, try using crystal skulls - flameskulls made of crystal, possibly with a damage type shift - I'd go lightning.

Anything with a shell can be silicate: try using ankhegs or bulettes as sappers. A Barlgura or Goristro would make an excellent siege engine.

Helmed Horrors - these are damn fun. Load them up as psychic-proofed, and go to town.
Gargoyles are good basic flyers, but lack any real ranged ability. Give 'em 90-foot range eye beams that roughly do claw damage, and you can leave the CR as-is.

Gas Spores: it's a "dumb" energized crystal sphere (or whatever shape you decide to make the body) that releases psychic slivers that will slowly kill the infected creature, and eventually grow more spheres. It's like a self-replicating claymore mine.

Crystaline Ropers. Good defense emplacements. You could accelerate the lifecycle and use piercers as artillery, which grow into full ropers in a matter of turns, creating a persistent damage zone.

Stone and Iron Golems, if you're feeling mean.

[Unoriginal]

Why not go for Duergars?