Thank you for both of your inputs. I really love the scorpion's grasp on the kraken. You can Spider-man your way from enemy to enemy, grappling lots of them from 120 ft and keeping them with you as you roam the battlefield. Really, it almost seems like this was designed for the kraken to use (even though the physics of this are way less than unquestionable). Why would you take Kraken Mantle anyway? A Shape Soulmeld (Girallon Arms) is much more efficient and doesn't take any class level from you (note that the totem chakra is not really interesting, since you'd much rather grab from 120ft away than wait for them to come nearer than 30 ft to use the supplementary arms). Plus you have a high essentia capacity with all your HDs so you can get interesting bonuses.

And I didn't know of Black Blood Cultist, but those are some really nasty damages (10d6+6d8+128=190 damages per grab if you have 18 Str before modifiers), per grab. Still, this is 20th level. If anyone doesn't have permanent Freedom of Movement at this level, that's more their bad than you being broken. But I agree that there shouldn't be such a power bump from just one level (except if that level is the 21st).

In the end, with three people moving to increase the kraken's number of HD, I think 12 RHD is good. Less versatility than a Storm Giant, but so much more damage if it can pull off its trick. I even wonder if 13 RHD would not be warranted. One of the major things that pulled the kraken back is its lack of mobility, and Scorpion's Grasp remedies that pretty well. More than well in fact. You would still be worried about Freedom of Movement, and Antimagic Field doesn't help that. It is a 10 ft emanation, and you try to grapple from 120ft away. But at this mid-high level, there is little chance that every one of your enemies will have it, so you can grab one of them, pull yourself to the opposing party, and pummel the FoM ones with your 8 other natural weapons. What do you think?