There are holes in the lakes

The Underworld

A young man from up north came to our village once. He did not believe our lake had a hole. His power was Loon, who dives everywhere in the ocean and doesn't fear the water. He needs Otter to use those holes! We saw him reach the bottom, but he never came back up.
His family found his body later, out on the saltwater. Our cousins there told us this. He didn't have a head. His head was gone! But there was an image of Loon tattooed on his chest, and that is how they knew him. It is a long way from our lake to the saltwater. But there is a hole in the lake.
-Kulamish, traditional

Of the routes to the Underworld, the various holes in the lakes and oceans are by far the quickest. These occur in the deepest, darkest places, beyond the reach of most divers (though the otters and their allies are said to be capable of the journey). From these hidden paths many strange and dangerous things escape into the world, bursting forth onto the surface to harass the villages from an Underworld that teems with bizarre life.

Occasionally these routes are forced to the surface through unknown means, heralded by thunderous cracking. Those investigating the openings describe descending into a world of labyrinthine tunnels where corals, pipeworms, and sponges loom over wandering fishes and molluscs illumed by the blooming of luminescent polyps, and fish wearing the shape of men contend with empty ghosts for scraps of clamshell and warmth. Many of these creatures drift by on unseen currents exactly as aquatic beings; others walk the tunnels unencumbered as if in open air.

Perhaps more dangerous are the persistent illusions that haunt the minds of foreign beings here - phantoms of dead loved ones that dance just at the edge of sight, walls that twist and move as one watches, and masses of chittering crustaceans staring back from the dark. These figments are found in every direction and are only dispelled by bright, terrestrial light; the soft glow of the Underworld's bioluminescent inhabitants seems only to reinforce their reality. Eventually, they could drive one mad.

There are inexplicable structures in those depths as well. Lurking within bizarre configurations of rooms carved from the rock are jealous spirits who squat over caches of precious metals - coins, tools, and weapons of forgotten make. These beings are not relatives, so natives think it only right that their copper be taken and put to better use, and every Company man knows that Made Beaver tokens (good only at the Company store) do not spend nearly so well as captured gold.

And somewhere beyond or within those tunnels are realms and sub-realms even more distant from human experience. Exploring deeply, one might stumble upon the Grave of the Orcas, the manses of great spirits like Copper-Maker and Tiyulhabaxad, or the innumerable Fading Caverns of the Dead where ghosts wait to forget the living.

Special Rules of the Underworld
Movement: All creatures both native to the Underworld and moving primarily by an innate swim speed (or that are otherwise water-adapted) may move, breath, and perform all other functions within the Underworld as if underwater. PCs and other humanoid creatures (those with the capacity to walk normally) react to the Underworld as if it were air-filled.

Vision: PCs and their allies cannot see properly unless possessed of a bright, terrestrial light source. Characters without access to proper light must make a saving throw each turn in order to act without reference to figments and mirages. Denizens of the Underworld suffer no such restriction, seeing without penalty.

Space: Distance traveled in the Underworld does not necessarily correlate to the same distance above. Portals to the surface 100 feet apart below could open into lakes separated by dozens of miles.

Some Denizens of the Upper Tunnels

Number Appearing: 2-12
Armor Class: 6 [13]
Hit Dice: 1+1
Attack: Claws (1d6)
Special: None
Move: 9 / 12 swim
Morale: 7
Size: Medium (6' tall)
Alignment: Chaos
Intelligence: Man-like

Dzegwa are rubbery fish-people native to the Underworld. They appear as tall, muscular humanoids with fine scales, wide mouths filled with rows of sharp teeth, and full heads of long, perpetually-wet black hair. Their hands and feet are webbed and possess digits tipped with vicious claws.

Dzegwa are well-known thieves of food and man-made objects, lurking just beneath the surface of seas and lakes near the villages of men. Especially reviled is their habit of kidnapping human children to the Underworld, where through a series of unfathomable tortures they transform them into waqwiwsu. Dzegwa speak with voices that are distressingly human, but only in older languages of local provenance.

Dzegwa attack with tooth and claw; they carry no other weapons. Generally they prefer to thieving and fleeing to standing and fighting. If feeling overconfident or forced into a fight, they will always attempt to hide and ambush, where possible.

Treasure: Stolen tools, clothing, and shiny objects.

Number Appearing: 3-18
Armor Class: 7 [12]
Hit Dice: ½
Attack: Brawl (1d6)
Special: Leap
Move: 6 / 9 swim
Morale: 6
Size: Small (3'-4' tall)
Alignment: Chaos
Intelligence: Child-like

Children kidnapped by the dzegwa are transformed into these bloated, frog-like beings through unknown processes. Waqwiwsu retain the childish stature of their previous lives but increase greatly in strength, girth, and viciousness, having a profound hatred for all life (especially their own). Their skin is green or green-black, their eyes dead and unblinking. Waqwiwsu cannot speak, emitting naught but horrible croaking, and give only the vaguest inclination that they can understand speech.

When a waqwiwsu is slain, black mud pours from its wounds and orifices, slowly returning it to a bruised but human shape. This reversion can also be achieved by hanging a waqwiwsu upside-down and allowing this mud to slowly dribble from its mouth, but that process is equally fatal. There is no known way to recover alive the child a waqwiwsu once was.

Waqwiwsu typically attack by battering enemies with their swollen fists, but in times of distress or agitation they may use their froggy legs to effect a great leap. This allows an immediate melee attack with +2 to hit from distances of up to 30 feet. Unfortunately, due to their unstable, bloated nature, the waqwiwsu must then make a saving throw or experience an immediate dermal rupture, showering all near the point of impact with a horrific spray of mud and guts. This occurs regardless of whether or not the leap attack hits; it is always fatal.

Treasure: The occasional coin, bauble, or trinket mixed in with their mud.

Rotting Ghost
Number Appearing: 2-12
Armor Class: 7 [12]
Hit Dice: 1
Attacks: Grasp (1d6 cold) or strike (1d6)
Special: Undead (immune to sleep, charm)
Move: 9
Morale: 7
Size: Medium (5'-6' tall)
Alignment: Chaos
Intelligence: Child-like

The path to the realm of the dead is long and torturous. The most pathetic of souls - those with no sense of responsibility or honor - simply choose not to go there. These wretched corpses loiter around the Underworld ambushing living beings in an attempt steal their warmth, the only relief they can achieve from the pain of being dead.

Rotting ghosts appear as decomposing human bodies with oversized wooden prosthetics compensating for limbs and features that are gradually rotting away. Such pieces are of variable quality, though most are painted with bright colors in comic imitation of life. Whether through vanity, self-denial, or some external impulse, all rotting ghosts are compelled to make these hopeless repairs. Eventually their bodies wither away completely, allowing them to shame their families no more.

In combat, rotting ghosts attempt to scratch with their nails and bite with their teeth. While this causes little physical damage, each contact allows them to steal a little of the innate warmth emanating from their target's soul. Those without proper extremities instead club their opponents with giant wooden hands, needing to bludgeon prey into submission before any warmth can be extracted.

Rotting ghosts still possessed of their vocal apparatus may speak the languages they knew in life, though most only mutter and mumble paranoid and self-obsessed delusions. Those without mouths or tongues just moan unintelligibly. In either case, they are difficult to reason with. If it is brought up, they refuse to acknowledge that their bodies are not whole.

Treasure: Various wooden extremities of questionable artistic merit.

Ripworm, small
Number Appearing: 2-12
Armor Class: 6 [13]
Hit Dice: ½
Attack: Rasp (1d6)
Special: Grapple
Move: 24 swim
Morale: 7
Size: Small (3'-4' long)
Alignment: None
Intelligence: Non-intelligent

Ripworms are segmented invertebrate predators that undulate through the Underworld with great rapidity. The entire length of their brightly colored, flexible bodies are lined with rough bristles used as both weapon and locomotive aid.

Ripworms feed on most of the fleshly beings of the Underworld. Though they possess mouths, ripworms attack by rasping flesh off their prey using the chitinous protrusions on their sides, scraping along the edge of their target's bodies as they rush past. A ripworm that scores a natural 20 for its attack roll enters a grapple with its target, wrapping its body around them and dealing damage automatically each turn thereafter. While in a grapple, a ripworm may not be targeted by weapons without endangering its victim (equal chance of hitting either). Any character may spend a round removing the ripworm, though accomplishing this with bare hands involves taking 1 point of damage in the process.

Treasure: A properly prepared ripworm corpse (requires leatherworking tools and expertise) forms a flexible rasp or improvised saw that can be used to cut and shape wood or soft stone.

Drift Anemone
Number Appearing: 1-3
Armor Class: 7 [12]
Hit Dice: 2+2
Attack: Sting (Paralysis)
Special: Multi-attack, Paralysis, Digest
Move: 6 swim
Morale: 10
Size: Small (3'-4' diameter)
Alignment: None
Intelligence: Non-intelligent

A drift anemone resembles a floating mass of smooth, stretchy tentacles, all probing about for food. Concealed within the mass is a rubbery, polypous body that propels the anemone through rippling contractions. They are of variable coloration, though bright blues, reds, and oranges are most common.

Drift anemones may attack all opponents within melee reach each round, probing outwards with dozens of writhing tendrils. Any creature hit by the anemone's sting must save or be paralyzed for 2d6 turns. Creatures significantly larger than human size gain a +4 bonus on this save.

When there are no more moving targets nearby, the anemone will attempt to digest the nearest victim. To do this the anemone must effectively turn itself inside-out, folding its polyp around the prey with tentacles tucked inside. A feeding drift anemone thus resembles a lumpy figure enclosed in a thick, fleshy sack. The process of enveloping a man-sized creature takes 1 turn. Once within, the prey takes 1 point of damage per round until dead and is only fully consumed after 1-3 turns. Only bones are expelled when the anemone returns to its active state.

Treasure: A character with an Intelligence or Dexterity of 15+ may spend a turn attempting to extract a paralytic agent from a drift anemone corpse (1 in 6 chance of success). This will be a solid piece of tentacle, not a liquid, and it becomes useless within a week. Only 1-3 viable samples may be acquired per corpse.

Pulse Jelly
Number Appearing: 4-24
Armor Class: 7 [12]
Hit Dice: 1 hit point
Attack: Sting (1 point)
Special: Bioluminescent flash
Move: 12 swim
Morale: 5
Size: Tiny (1' across)
Alignment: None
Intelligence: Non-intelligent

Pulse jellies are one of the more common Underworld lifeforms, feeding off the smaller crustaceans and fishes and providing food to larger organisms in turn. Their translucent, bell-shaped bodies give off a slight iridescent glow as their pulsating motions propel them through the Underworld.

Pulse jellies are mostly harmless, being generally non-aggressive towards larger beings. Contact with their tendrils does provoke a sting, but they are rarely incited to attack on their own. Their true danger lies in their ability to produce blinding flashes of light when spooked. Pulse jellies can usually be bypassed if one is careful - a turn of slow movement allows a 4 in 6 chance of sneaking through a group of them - but any sudden motion or loud noises will trigger their flash. All seeing creatures within 60 feet of such a flash are blinded for 2d6 rounds (a successful save halves the duration). Additionally, flashes occurring in non-enclosed spaces open to the tunnels immediately provoke 1-3 wandering monster checks.

Treasure: A character with an Intelligence or Dexterity of 15+ may spend a turn attempting to fashion a crude light source from a pulse jelly corpse, provided its blinding flash was not used recently (4 in 6 chance of success). This requires taking 1 point of damage unless sturdy gloves are used. Light given off is equivalent to a torch and lasts for 2d6 turns. A character with similar scores may instead attempt to construct a flash bomb from the corpse (1 in 6 chance of success), though such a contrivance would need to be triggered by hand and will only work if used within 2d6 turns.

Abbreviated Stat Blocks
Dzegwa (AL C, HD 1+1, AC 6 [13], MV 9 / 12s, ML 7)Rubbery fish-people that steal children. Attack with claws.
Waqwiwsu (AL C, HD ½, AC 7 [12], MV 6 / 9s, ML 6): Kidnapped children turned into bloated frog monsters. Leaping brawlers. Leak out mud on death. 30' leap attack (+2 hit, save or explode).
Rotting Ghost (AL C, HD 1, AC 7 [12], MV 9, ML 7)Pathetic corpses who carve sub-par wooden replacement parts to hide their rotting flesh. Grasp steals warmth or clubs with wooden hands. Undead.
Ripworm, small (AL N, HD ½, AC 6 [13], MV 24s, ML )Horrifyingly fast, swimming worm that rasps off flesh with spines. 3'-4' long. Grapples on natural 20 (auto damage each turn, removing deals 1 damage).
Drift Anemone (AL N, HD 2+2, AC 7 [12], MV 6s, ML )Floating mass of tentacles that shoot paralytic darts (no damage, save or paralyzed 2d6 turns). Attacks all in melee range each round. 1 turn to engulf paralyzed prey (1 damage per round, digest in 1-3 turns).
Pulse Jelly (AL N, HD 1, AC 7 [12], MV 12s, ML ): Bioluminescent jellyfish. Sting deals 1 damage. Flash blinds 2d6 rounds & provokes 1-3 wandering monster checks (save for half duration, 1 turn allows 4 in 6 chance to sneak around).