Cleon
Legend
So the (b) could be equivalent to restrained or grappled . Boils down to how much importance there is to shoehorning it to an existing condition or improvising. Alternate would be like paralyzed but can cast spells with verbal components and talk.
Neither restrained or grappled seem right since the target can still take any kind of action except move, although they have disadvantage on attack actions in the case of restrained.
I'm more inclined to make it a new condition based on paralysed. The medical term for paralysis that affects the limbs but not the head is tetraplegia, so maybe we could call it that?
As for the effects, how about "the creature can't take actions or reactions requiring their body or limbs, but may speak and perform actions that only involve their head or neck, such as verbal-only spells and breath weapons. The creature automatically fails Strength and Dexterity saving throws and attack rolls against the creature have advantage."
What are we doing about the original breath weapon causing permanent loss of Dex, Str and Con? You've got it causing Exhaustion with a Con Save in the current draft, but there is precedent for ability draining attacks in 5E with the Shadow and the like:
5E SRD said:Strength Drain: Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 9 (2d6 + 2) necrotic damage, and the target's Strength score is reduced by 1d4. The target dies if this reduces its Strength to 0. Otherwise, the reduction lasts until the target finishes a short or long rest. If a non-evil humanoid dies from this attack, a new shadow rises from the corpse 1d4 hours later.
So interpreting the Gurgotch's attack into that form the "damage" would be something along the lines of:
"…the target's Dexterity score is reduced by #d#, its Constitution score is reduced by #d# and its Strength score is reduced by #d#. The target is paralysed for 1 hour? if this reduces its Dexterity or Strength to 0 and dies if this reduces their Constitution to 0, otherwise it must succeed at a DC ## Constitution saving throw or become paraplegic for #d# minutes."
Having it do three sorts of ability damage seems excessive, so I was thinking we could only have it affect one ability (Dexterity) or two (probably Strength and Dexterity) to reflect its paralytic affect. The latter can be achieved by snipping out the red text from the above.
I'm not keen on making the ability damage Permanent like the original - that's way too nasty for standard 5E. Maybe allow it to return with short or long rests? Perhaps with Con saves to recover a point or #d# points with a short rest and the usual full heal for a long rest?
(a) The original was just on one creature in front of it, but that seems a bit silly for a huge creature to parp out such a tiny weapon. So maybe is better as a cone - thinking as it shoots out of its snout. So like a hell hound maybe.
The original text says "it exhales a noxious gas which acts on the central nervous system of a victim. All caught in the cloud must ‘save against constitution’ or lose all conscious muscular control from the neck downwards (in effect, they are paralysed)" so it's a cloud that can affect more than one creature per breath. It wouldn't have the "All" if it only breathed on one creature in front of it.
The Comments by Fiend Factory editor Don Turnbull in White Dwarf #14 points that out, asking "What range and area of effect has the breath weapon?"