The monsters get deadlier (or weirder, or both) as we get to CR 30. I'll see you tomorrow with the CR 25 monster!
The colors and general features of this massive spined, multi-legged worm shift from moment to moment. Images of familiar yet distorted memories appear at random around the creature.
Giant Hallucigenia Worm CR 24
CE Huge aberration
Init +12; Senses darkvision 60 ft.; Perception +42
AC 42, touch 20, flat-footed 30 (+12 Dex, +22 natural, –2 size)
hp 527 (34d8+374)
Fort +24, Ref +23, Will +24
Defensive Abilities lies become reality, spines; DR 20/cold iron and lawful; Immune mind-affecting effects
Speed 40 ft., burrow 30 ft., fly 90 ft. (average)
Melee 4 claws +35 (3d6+9 plus grab), spine +36 (2d6+11/19–20 plus poison)
Space 15 ft.; Reach 15 ft.
Special Attacks diverting attack, hallucigenscape, nightmare curse, poison, rend (2 claws, 3d6+13)
Spell-Like Abilities (CL 42nd; concentration +47)
At will—greater oneiric horror[OA] (DC 23), mass synesthesia[OA] (DC 25), mirror image, screen, veil (DC 24)
3/day—quickened greater oneiric horror[OA] (DC 23), quickened mirror image, quickened veil, weird (DC 27)
Str 28, Dex 35, Con 32, Int 20, Wis 21, Cha 23
Base Atk +31; CMB +43 (+47 grapple); CMD 60 (can't be tripped)
Feats Agile Maneuvers, Combat Expertise, Critical Focus, Deceitful, Great Fortitude, Greater Feint, Greater Spell Focus (illusion), Impaling Critical (spine)[UC], Improved Critical (spine), Improved Feint, Improved Impaling Critical[UC], Quicken Spell-Like Ability (greater oneiric horror, mirror image, veil), Spell Focus (illusion), Weapon Finesse[B], Weapon Focus (spine), Weapon Specialization (spine)
Skills Bluff +44, Disguise +44, Fly +22, Knowledge (arcana, local, planes) +39, Perception +42, Sense Motive +39, Spellcraft +42
Languages Aybssal, Aklo, Celestial, Common, Undercommon; telepathy 180 ft.
Diverting Attack (Ex) As part of a full attack action, a giant hallucigenia worm can forgo one of its claw attacks to make a Bluff check to feint an opponent.
Giant Hallucigenia Worm Poison (Ex) Spine—Injury; save Fort DC 38; frequency 1/round for 10 rounds; effect 1d8 Wis drain and victim is confused for 1d4 rounds (cumulative); cure 3 consecutive saves.
Hallucigenscape (Su) Once per day, a giant hallucigenia worm can put a number of creatures equal to half its Hit Dice and within range of its telepathy into a strange mindscape. This operates similarly to microcosm[OA], but has no limitation on the total Hit Dice it can affect, and creatures with 16 or more HD that make their saves are still in the mindscape for 1d6 rounds. A DC 33 Will save partially negates this effect as per the spell (and with the above caveat). While creatures are in the mindscape, the worm spend a standard action to populate the mindscape for a particular creature with an exact duplicate of itself (but at full health) or a CR 20 (or lower) creature. If the worm chooses something other than a duplicate of the affected creature, the creation gains DR 10/— and the worm’s lies become reality ability (the save DC is lowered to 29, however). If a target creature dies in the mindscape, it returns to reality, unconscious yet stable, and with a negative number of hit points equal to its Constitution – 1. Even if the creature is healed to 0 or positive hit points, it must succeed at a DC 33 Will save to awaken; otherwise, it awakens after an hour. If a target creature manages to defeat its opponent in the mindscape, it returns to reality immediately, but with the wounds sustained while within the mindscape. Creatures that failed their initial save (or did not receive a save) are subject to the worm’s nightmare curse. This is a mind-affecting effect. Intelligent creatures do not benefit from an immunity to mind-affecting effects against this ability; they instead receive a +4 insight bonus on their Will saves, and, if they succeed, they ignore the effect entirely. The save DC is Charisma-based.
Lies Become Reality (Su) A creature that attacks a giant hallucigenia worm, or targets it with or includes it the area of effect of a spell that deals damage, must succeed at a DC 33 Will save or become convinced the attack or spell has no effect on the worm, causing the worm to take no damage. Further uses of the same attack or spell grant the creature a cumulative +2 insight bonus on its Will save. The save DC is Charisma-based.
Nightmare Curse (Su) save Will DC 33; frequency 1 day; effect 1d6 Wis drain, the victim is fatigued, and it is unable to regain arcane spells for the next 24 hours. A creature that attempts remove curse or any similar spell to remove the nightmare curse and fails is subject to this curse.
Screen (Sp) A giant hallucigenia worm only requires 1 round to use its screen spell-like ability. When it uses this spell-like ability, any previous instances are dispelled.
Spines (Ex) Any creature that makes a melee attack against a giant hallucigenia worm must succeed at a DC 36 Reflex save or be struck by the worm’s spines, taking 2d6+6 points of damage and becoming subject to its poison. Using a reach weapon does not endanger the attacker in this way. The save DC is Strength-based.
Giant hallucigenia worms spend most of their near infinite lives dreaming of civilizations predating even the most ancient mortal nations. When they awaken from their dreams, they spread chaos and incite violence by inflicting their dreams upon others. The worms create compartmentalized mindscapes from images in their minds and place their victims alone in these mindscapes. Monsters or hostile duplicates of the victims populate these surreal locations and attack the victims while taunting them about their failures. The worms use their powers of illusion to disguise the monsters as their victims’ allies, fomenting strife between those who fell under the worms’ power. The mindscapes do not kill their victims, but generate recurring nightmares and create a profound distrust of others and of reality itself. Giant hallucigenia worms are fascinated by creatures that show resilience to their mindscapes and may seek to parlay with them rather than attack. They prefer not to avoid combat but they have the physical might to defend themselves, and their hallucinogenic properties make it extremely difficult to harm them.
Very few creatures have glimpsed the worm’s true form, but it is roughly 30 feet long and weighs nearly 8 tons.