Probably everyone has read the story of this hideous thing, the giant polychaete worm that popped up in an aquarium: Barry the giant sea worm. In any case, it's going to be making an appearance at some point. The photos of it filled me with such utter revulsion (childhood memories of being bitten by his littler cousins while I tried to hook them for use as fishing bait), that I'm inspired to make a mid-level monster from it.
And yeah, it's Chaotic. When you look that ugly, you've picked sides.
Worm, polychaete, giant
No. Enc: 1d6
Movement: 90' (30')
Swim: 150' (50')
Armor Class: 3
Hit Dice: 5
Hoard Class: VII
The giant polychaete worm is one of the most feared creatures in the sea. In appearance, it is as revolting a specimen of worm as can be imagined, with thousands of legs running down the sides of its 20' long body, each leg tipped with a single sharp claw. The head of the worm has a nasty set of mandibles, designed not just to bite meat off, but to tear through the flesh of its prey.
Once the worm has successfully bitten an opponent, it will hang on and continue chewing through its victim for each round thereafter, automatically doing another 2d6 damage each round. The victim has a +2 bonus to hit the worm while it is gnawing away at his chest, seeking out his still-beating heart for a mid-afternoon snack. It's hard to miss the 20' long worm hanging off your body, after all. After damage is applied each round, the victim may attempt a save vs. petrification to see if he can shake off the worm.
The giant polychaete worm requires a moist environment to live, but can leave the water for a few hours at a time with no ill effects. These creatures are typically found in the ocean, but are occasionally also found in underground pools and lakes, feeding on dungeon inhabitants. It is particularly vulnerable to fire, taking double damage from fire-based attacks.
Whatever treasure the worm has will be incidentals that it has consumed along with its victims, and adventurers will have to cut into its gut to retrieve the loot. This is time consuming, as it has a hard exoskeleton, and the gut extends the length of the creature's body.