Unless someone gives me very good reasons to change some of it, this is the final version of our jellyfish. Input on the CR is always welcome.
XXX
Gargantuan Vermin
Hit Dice: 23d8+230 (333 hp)
Initiative: -5
Speed: Swim 5 ft.
Armor Class: 21 (-5 Dex, -4 size, +20 natural)
Base Attack/Grapple: +17/+41
Attack: Tentacle +25 melee (2d8+12 plus 2d8 electricity plus poison)
Full Attack: 4 tentacles +25 melee (2d8+12 plus 2d8 electricity plus poison)
Space/Reach: 20 ft./15 ft. (100 ft. with tentacles)
Special Attacks: Digestive polyps, electricity, improved grab, poison, psionics
Special Qualities: Amorphous, blindsight 250 ft., damage reduction 15/-, humanoid symbiosis, levitate, vermin traits
Saves: Fort +23, Ref +2, Will +2
Abilities: Str 34, Dex 1, Con 31, Int -, Wis 1, Cha 14
Skills: Hide +13 (+3 if inhabited)*
Environment: Any aquatic
Organization: Solitary, hive (1 plus 2 - 20 monstrous jellyfishes) or inhabited (1 plus 1 - 8 humanoids)
Challenge Rating: ??? 17 ???
Treasure: None
Alignment: Always neutral
Advancement: 24 - 31 HD (Gargantuan); 32 - 69 HD (Colossal)
The xxx is a colony of polyps living in close symbiosis, similar to the man-o-war, but reaches the size of a small island. As large as it is, it is hard to see since it is mostly transparent except for purple veins running through its body and its enormous tentacles. A/n xxx produces some strange effect that causes smaller jellyfish to congregate around it, sometimes forming a strange hivemind capable of creating psionic abilities. A side effect of the gathering is the appearance of the normally subterranean oddities known as flumphs around the xxxs. While normal creatures would propably be annoyed by the flumphs, the xxx found a simple and elegant solution to this dilemma. It considers flumphs to be a delicasy. A few XXXs seem to be able to bond with humanoid creatures, allowing themselves to be inhabited.
Combat
An opponent can make sunder attempts against a xxx's tentacles as if they were weapons. A/n xxx's tentacles have 40 hit points. If a/n xxx is currently grappling a target with one tentacle, it usually uses another limb to make its attack of opportunity against the sunder attempt. Severing a/n xxx's tentacle deals damage to the xxx equal to half the limb's full normal hit points. A/n xxx usually has 8 tentacles capable of attacking, but only four can do so in a normal situation. If it loses all but 2 tentacles, it withdraws from combat. A/n xxx regrows lost tentacles within 2d4 days.
Digestive Polyps (Ex): A/n xxx can try to convey a grabbed opponent of up to two sizes smaller into one of its digestive polyps by making a successful grapple check. The "swallowed" creature takes 2d8+12 points of bludgeoning damage and 16 points of acid damage per round from the xxx's digestive polyp. A swallowed creature can cut its way out by using a light slashing or piercing weapon to deal 40 points of damage to the polyp (AC 20). Once the creature exits, the polyp releases all other creatures it holds; an opponent held by a different polyp must cut its own way out.
A Gargantuan xxx's digestive polyp can hold 1 Large, 8 Medium, 32 Small, or 128 Tiny or smaller opponents. The number of digestive polyps a/n xxx has is limited, but in most cases can be treated as unlimited.
Electricity (Ex): The tentacle attacks of a/n XXX deal an additional 2d8 points of electricity damage with each successful hit.
Improved Grab (Ex): To use this ability, a/n xxx must hit with its tentacle attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can attempt to convey the victim into its digestive polyps the following round.
Poison (Ex): Injury, Fortitude save DC 31, initial and secondary damage 1d6 points of temporary Strength damage.
Psionics (Sp): A/n XXX that is accompanied by monstrous jellyfishes (or similar creatures) can cannibalize the life force of these creatures to create some psionic abilities. As the XXX and the jellyfishes form a very primitive hive mind, it can use the abilities somewhat appropriately (the actual Intelligence of the XXX and the jellyfishes does not change). The numbers in brackets before the save DC notes the number of Hit Dice the XXX must cannibalize (treat as negative levels) from the jellyfishes to manifest the power (note that monstrous jellyfishes, as found in the Tome of Horrors, page 172, have 3 HD).
Cloudkill (10 HD, DC 17),
greater magic fang (self only, 6 HD),
ice storm (8 HD, DC 16),
major image (6 HD, DC 15). Caster level 23. Save DCs are Charisma-based.
If you have access to the psionic rules you could consider adding some of the following powers (replacing some of the above, if you wish):
Charm monster (6 HD, DC 15),
crisis of breath (6 HD, DC 15),
dissolving touch (8 HD, DC 16),
recall agony (10 HD, DC 17).
Amorphous (Ex): As a being made up of a large number of comparably small creatures, a/n xxx is not subject to flanking and critical hits.
Humanoid Symbiosis (Ex): In a few rare cases, a/n XXX might form a symbiosis with humanoids or monstrous humanoids instead of jellyfishes, allowing a family to inhabit and harvest food from its body. A typical XXX can support up to eight Medium humanoids (adjust the numbers accordingly to the size of the humanoids and the XXX). A/n XXX in a symbiosis with humanoids can utilize its psionic abilities, but instead of negative levels, it deals permanent Wisdom damage, distributed evenly among the inhabitants (affected creatuers can attempt a Will save DC 23 to reduce this damage to temporary ability damage). As a side effect, symbiotic humanoids can use mind-affecting effects on the XXX, circumventing its immunity to such effects. Rumours persist about a whole city of inhabited XXXs, but if it exists, it has yet to be found.
Levitate (Su): By inflating its float, a/n XXX can continually
levitate, as per the spell of the same name.
Vermin Traits: Immune to mind-affecting effects. Note that symbiotic humanoids can use such effects on the XXX (see humanoid symbiosis, above).
Skills: XXXs receive a +30 racial bonus to Hide checks, as they are highly transparent. Despite their size, they are extremely hard to spot. *An inhabited XXX is obviously easier to spot, and receives only a +20 bonus to Hide checks
Name Suggestions:
Jellyfish Lord
Monstrous Jellyfish Lord
King of the Sea
Merfolk Eater
Tentacruel
Megabloat
The Dread Sail
Otu
Living Island
Flesh Island
Ooze Island
Wave Behemoth
Jellyviathan
(I think those are enough for now, but I won't start the naming poll today

)