She also loses the claw attack and her speed. Of course, she gains the flop. I can see CR +2 if you think Flop is equivalent to being able to move and claw, but I'm not entirely convinced. As you say, it would be pretty easy to stay back and used ranged attacks. In other words, I doubt a party is going to get too many people pinned.
Yes, the immobility certainly ought to drop the CR by at least one or two.
Shall we say CR 5 for the time being?
Let's see... the stat block in question would look something like:
Isopterite Primary Queen (Colossal Monstrous Humanoid;
Hit Dice: 16d8+128 (136 hp);
Init: –1;
Speed: 0 ft.;
AC: 15 (–8 size, –1 Dex, +14 natural), touch 1, flat-footed 15;
Base Attack/Grapple: +16/+46;
Attack: —;
Full Attack: —;
Space/Reach: 30 ft./ 20 ft.;
Special Attacks: Flop [
DC 26, then either crush for 2d12+21 damage or Bull Rush with +20 check modifier];
Special Qualities: Darkvision 60 ft., hive mind;
Saves: Fort +15, Ref +9, Will +13;
Abilities: Str 39, Dex 8, Con 26, Int 4, Wis
13, Cha 11;
Skills: Knowledge (geography) +5, Knowledge (history) +5, Knowledge (local) +6, Knowledge (nature) +6;
Feats: Great Fortitude,
Iron Will, Open-Minded (
×3), Snatch;
CR: 5)
I put both its ability advancements in Wisdom, since that seemed appropriate for a "repository of the hive's knowledge" idea we're running with.
You know, there's no point in keeping the Snatch feat, since the Primary Queen doesn't have claw attacks like the basic Queen. Should we change it or leave it in? It'll help keep the CR low if she has a "useless" feat.
Alternatively, we could give her a bite attack that substitutes for the original's claw - presumably she still has a functional mouth. Maybe give it the same damage as her claw attack would do if she had one?
e.g.:
Attack/Full Attack: bite +22 melee (4d6+21);
If we do all of the above, we'd want to tweak the Colossal Queen's combat description. This ought to do:
An isopterite queen of Colossal size is so swollen and cumbersome has a speed of 0 ft. A Colossal queen loses the claw attack of her smaller sisters and gains a bite attack in its place that does the same damage. She also gains a special attack called Flop, described below. A stat block for a typical Colossal Isopterite Queen is appended to the end of this subentry.