Ok - then maybe just leave as speed=0 - with a note in the description that they can move very slowly if needed but much too slow to be perceptible in combat? Not sure how/why had perception like that. tweaked now. Ditched false appearance (was pinched from Galeb Duhr)
Updating the
Chthon.
It's still Medium in your latest update, I thought you'd agreed on Large like a standard Earth Elemental?
I'd be OK giving it
False Appearance or
Stony Camouflage if you want to make it stealthy, but I'm equally fine without it.
Oh, if they have Challenge 5 worth of critters at their command shouldn't that be included in their CR? That would make them at least Challenge 6.
Now let's talk Mental Stats.
The original had High Intelligence, which in AD&D is Intelligence 13 or 14. It's two steps up from Average intelligence so I'm thinking INT 14 so it gets a +2 modifier. Your latest draft has INT 15, which also gives a +2 but is in the next higher Exceptional Intelligence bracket in AD&D, the next rung up.
If we kept the +1 for WIS, +2 for CHA ability modifiers of your draft and followed the usual "three odd scores, three even scores" rule, then the WIS needs to go up one to 13 but we can keep the CHA at 15:
Mental Chthon #1: INT 14 (+2), WIS 13 (+1), CHA 15 (+2)
I like Wisdom as its lowest mental stat, considering it "resents animal intelligence with paranoid fervour" I'd think it might be lower than that since the thing may be smart but it's pretty crazy. I'd consider having it WIS 10 like a regular Xorn or Earth Elemental and lowering the INT to the Highly Intelligent starting value of 13:
Mental Chthon #2: INT 13 (+1), WIS 10 (+0), CHA 15 (+2)
Of the two I'm leaning towards #2.
Might want to give it
Saving Throws in Wisdom and
possibly Intelligence and/or Charisma too. I imagine its mind is as stubbornly resistant as a rock.