This horse-sized snail has a mottled, brown shell and four tentacles on its head, each ending in a humanoid skull.

Ghastropod (CR 3) XP 800
NE Large undead Init –1; Senses blindsense 30 ft., scent, tremorsense 60 ft.; Perception +6 Aura stench (10-ft. radius, Fort DC 15 negates, sickened for 1d6+4 minutes)
--- AC 18, touch 8, flat-footed 18 (–1 Dex, +10 natural, –1 size) hp 39 (6d8+12) Fort +4, Ref +1, Will +6 Defensive Abilities corrupt magic; Immune undead traits; Resist fire 20
--- Speed 10 ft., climb 10 ft. Melee 4 slams +7 (1d6+3 and paralysis) Space 10 ft.; Reach 10 ft. Special Attacks foul mucus, paralysis (1d4+1 rounds, DC 15)
--- Str 16, Dex 8, Con —, Int 5, Wis 12, Cha 14 Base Atk +4; CMB +8; CMD 17 (can’t be tripped) Feats Power Attack, Weapon Focus (slam) Skills Climb +15, Perception +6, Stealth +0 Languages Flail Snail (sign language, slime writing, cannot speak) SQ corrupt magic, foul mucus, retraction, suction
--- Corrupt Magic (Su) Anytime a spell targets a ghastropod, there is an 80% chance that it affects all targets as if it were an inflict spell of the same level (maximum 8th) instead of producing its normal effects. Only spells that directly target the ghastropod are warped; area effect spells are not affected. Foul Mucus (Ex) A ghastropod is coated with slime that renders it immune to the mucus of living flails snails. As a free action, a ghastropod can excrete a trail of foul mucus which covers its space and lasts for 10 minutes. Neither slippery nor sticky, this mucus doesn't affect movement or otherwise create a barrier. It instead produces noxious fumes; any living creature entering a space covered with foul mucus is affected as if standing within the ghastropod's stench aura. Foul mucus is not flammable. Retraction (Ex) A ghastropod can pull its fleshy parts back into its shell as a swift action, increasing its natural armor bonus by +6, but it cannot move or attack while retracted. It can return to normal as a free action. Suction (Ex) A ghastropod's foot adheres to surfaces so well that its 10-foot climb speed applies even on perfectly sheer surfaces and ceilings, with no chance to fall off unless the foot is actively pinned and peeled away as part of a grapple.
--- Environment underground Organization solitary or pair Treasure shell worth 400 gp

Also known as ghastropods, undead flail snails are rare creatures produced by the flail snail's ability to warp magic. On very rare occasions, a spell targeting a flail snail is not warped in the normal manner, instead producing an effect which instantly transforms the flail snail into a ghastropod.

In contrast to a living flail snail, a ghastropod has a monochromatic shell displaying various shades of brown, and has no ability to control the shape or consistency of the mucus it excretes. The ghastropod's tentacles resemble those of an ordinary flail snail, but the ghastropod compulsively covers the tips of these limbs with ornamental humanoid skulls, giving them a distinctive and disturbing new appearance.

A typical ghastropod looks to replace its morbid, bone jewelry at least once a week, harvesting skulls from murdered humanoids as needed to accomplish this goal. Favorite targets include families of four, whose thematically-linked skulls are considered particularly fashionable.

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