At your discretion, you can share the following information with the party. This is encouraged if this adventure is being used as a oneshot as opposed to as part of a larger adventure.

Long ago in the depths of the Underdark, a drow hermit, living within the outskirts of the tunnels surrounding Menzobarrenzan, located a vast network of tunnels known as The Wormwrithings. The tunnels came about as the aftermath of Purple Worms’ travel through the Underdark, with some stretching from the greatest depths of the Underdark all the way to the surface. Within one such tunnel, the hermit found a tiny, young Purple Worm.

Taking pity upon the runt, the hermit sought out a natural spring, a place where the soil would be soft and full of nutrients. Unbeknownst to the drow, the spring he found rested within a cavern where the Faerzress - the unusual magical energy which pervades much of the Underdark - was quite potent.

Over several months, the Purple Worm compounded in size until it reached the point that began to dig a Wormwrithing of its own. To begin, it consumed the cavern it had grown in, including the magical spring. The hermit, upon returning to the place the cavern once rested, was awestruck by the size of the tunnel he found, which was nearly 120 yards in diameter.

Over the centuries of its life, this Purple Worm became a local legend to denizens of the Underdark. Known as The Dire Worm, the stories which accompany it include small outposts disappearing overnight, miners striking an unbreakable purple wall, and travelers walking unknowingly into its belly, confusing its agape mouth for a cavern.

Despite its size, The Dire Worm is difficult to find in the hundreds of miles of the Underdark. The gargantuan tunnels it burrows are not stable for their size, and will usually collapse within a matter of days. Despite this danger, many who stumble upon one will quickly scour its length for the worm’s secretions, which include gems and precious metals of impressive size and quality. The Dire Worm will also spend periods of time in hibernation, curled up in a knot. Rarely, it has even been known to breach the surface, sometimes consuming a small village or a few acres of forest before burrowing downward once more.

A Linear Adventure

This dungeon is built upon the insides of a larger-than-average Purple Worm, with the party trying to find a way to either kill The Dire Worm or escape. Either way, progressing through the dungeon is going to follow a mostly linear path, although there will be opportunities for branching out. Owing to its size and the magical effects of the underground spring, this Purple Worm has almost twice as many internal organs and parts as a normal one. From this magic as well as the strange properties of many things it has swallowed over the years, many of its organs have been warped in strange ways. 

A Ticking Clock

When the party is first swallowed, The Dire Worm will have just begun a new cycle of digestion. As this cycle continues, its stomachs will begin to fill with acid. If the party cannot escape the stomach to which they are swallowed, they too shall be digested. Should they escape, they will need to progress further down the body of The Dire Worm, as the now-filled stomach will prevent them from returning in the direction they came. The stomach fills after 1 in-game hour. 

If running as a one-shot, for added tension you could add a real world timer, setting it to somewhere around 2-4 hours depending on how much time you want to give the party. If the party cannot escape by the time the timer goes off, it is assumed that 1 in-game hour has passed, and the party fails. 

Thick-Skinned

Owing to its size and the magical energies of the magical spring held within its body, The Dire Worm’s internal flesh is robust. A player will not be able to “dig” their way out of the body by cutting into the walls of its stomach. That being said, there will be locations within its body where the flesh or organs may be weaker and susceptible to damage. It would likely be a good idea to be upfront with the players about these facts, especially if using a timer. 

Stomach Acid

A creature which starts its turn inside of the stomach acid takes 6d6 acid damage. Thankfully, there are pieces of undigested rock that the party can move across as stepping stones to avoid danger. Even if not explicitly shown on the map, assume these stones are present throughout the stomach.

 Whenever a creature attempts to move on its turn, it must attempt either a Strength (Athletics) or Dexterity (Acrobatics) check at the beginning of its movement. The DC is 13, and on a failed check the creature falls into the acid and its movement speed is reduced to 0 for the remainder of the turn. If the creature takes the Dash action, it makes this ability check before moving again. A creature can also choose to move carefully, reducing its speed by half until the end of its turn, in which case it makes the ability check with advantage. A creature can use its action to pull another creature out of the acid. The acid takes a little time to begin digesting, and a creature can avoid the damage if it can get out before the start of its next turn.

The acid slowly rises within the stomach. For every 6 minutes that pass in game, increase the DC for navigating the rocks by 2, to reflect the disappearance of some of the stones. If you have a real-world timer, divide the total time into tenths and increase the DC each time one tenth of the time you have allotted passes. After 60 in-game minutes pass, the party can no longer avoid the acid, and takes the damage every turn.

If the party exits the stomach before the acid rises all the way, continue to track how its level is rising, as there will be certain areas revealed to convince the party to return to the stomach. 

Incoming! Even after devouring the party, The Dire Worm continues its gluttonous rampage, consuming rocks, trees, and even buildings.  For each minute that the party spends within this stomach, each creature must make a DC 13 Dexterity saving throw to avoid the debris. A creature takes 3d8 bludgeoning damage on a failed save, or half as much on a successful one.

Better Out than In. If a member of the party has access to a climb or flight speed, whether by natural ability or magic, they may attempt to move back up the throat. If this route is taken, have them make the same Dexterity saving throw for Incoming! at the start of each of its turns and at disadvantage. The total distance it must travel is 150 feet, after which point it arrives in the Gizzard (outlined later).

Some oozes from the underdark have taken up living symbiotically with The Dire Worm. The Black Puddings are naturally immune to The Dire Worm’s stomach acid and will usually hunt down organisms to consume for itself, aiding in the worm's digestion. The Gray Oozes are less comfortable, as the potency of the worm’s acid can be too much for them.

A large pile of rubble, some half-digested trees, and - resting above it all - an overturned carriage. These parts of the worm’s meal surely came from the surface. The acid continually bubbles around this small island of misplaced earth, which slowly groans and sinks further in. The carriage seems to be of fine design, with artistic filigree painted in gold around the exterior.

If a player has a passive Perception of 12 OR if a player succeeds on making a Perception or Investigation check before proceeding (DC 12), read the following

Moving among the rubble are five gray blobs which seem to be taking refuge from the acid. Lurking behind the pile are two large, black oozes.

If the party approaches the rubble or tries to attack, roll for initiative. The two Black Pudding can move freely through the stomach without repercussion, owing to their immunity to acid damage, but the five Gray Ooze can be harmed by the acid. Have the Gray Oozes make an Athletics check following the same rules for the stomach acid as the players. Note that the Gray Oozes are resistant to acid damage. The oozes will continue to fight until killed, but will not pursue the party if they leave this stomach.

Treasure: If the party searches the carriage, they can find a crystal decanter and two glasses worth 200 GP. The decanter is filled with fine wine. Stripping the carriage can garner five pieces of art and other luxuries worth 25 GP each. A successful DC 15 Investigation check will uncover a small chest hidden beneath one of the seats. Inside the chest is 100 gold pieces, two diamonds worth 300 GP each, and a small, ceremonial dagger. If the party successfully kills at least one of the Black Puddings, they can find Studded Leather of Acid Resistance within its remains. 

This pathway connects the upper two stomachs to the lower two.

This chamber continues deeper into the beast’s stomachs. Further beyond, you can see what appears to be another stomach to your left and right, and  you can see a large, gaping hole which leads into what you can only assume is the worm’s intestines. A means to exit the stomach now lies before you, yet it is blocked by the skeletal remains of three, large minotaurs. Wandering around with them are half a dozen humanoid skeletons, armed with heavily-degraded weaponry.

All of the skeletons here were animated by the magical artifact inside of the necromancer’s hut in Area S4. With their master now dead, they are wandering the stomach aimlessly, but will attack if they see one of the party members or another non-undead creature. The weapons carried by the six humanoid Skeleton have been eaten away by the stomach acid over time and have a -2 penalty to hit and to damage rolls. The three Minotaur Skeleton are not affected. If combat is initiated, six additional humanoid skeletons from Area S4 join the fight after 1d4 rounds (if they have not already been destroyed). 

The party can sneak into Area S4 or S5 with a successful DC 12 Dexterity (Stealth) check. The party could also choose to wait to see if the skeletons will wander away from the exit into either Area S4 or S5. For this route, the party may roll 1d20 for every 10 in-game minutes they are willing to wait. For the first roll, a value of 18 or higher on the d20 is needed for the monsters to wander away unaware. For each 10 minute period after that, the value needed decreased by 1, to a 17 or higher after 20 minutes, a 16 or higher after 30 minutes, and so on.

Once the party exits the stomachs through the hole, they are safe from the rising stomach acid. Continue to track any remaining time, as the party may choose to return to the stomach later. If time has elapsed and the stomach is full, returning is not possible. 

Resting in the center of this chamber is a large tower, seemingly veiled in magical energies to protect it from damage. Surrounding the hut is the remaining debris of several buildings and half a dozen wandering skeletons.

The tower belonged to a necromancer living in the underdark who was consumed, along with his tower, by the dire worm. The hut has the statistics of an Instant Fortress. The door is shut and the command word to open it is unknown, so the only means to enter is through climbing the ladder to access the trap door on the roof. The fortress is only resistant to acid damage and is slowly degrading through its magic veil; however, it will persist until the stomach is full. Once the acid reaches its maximum level after 1 in-game hour, the fortress will remain intact for 1d10 rounds, after which point it, and all of its contents, are consumed by the acid.

Bypassing the wandering skeletons without engaging in combat requires a successful DC 12 Dexterity (Stealth) roll. If combat is initiated, the three Minotaur Skeletons and six Skeletons from Area S3 arrive after 1d4 rounds. If the party enters the fortress, the skeletons do not pursue. 

Upon entering the upper most level of the tower through the trap door, the party finds itself in what appears to be a dusty old storeroom. Across from the ladder is another trapdoor leading further down the tower. If the party searches through the boxes, they can find any spellcasting foci and (non costly) spellcasting components they may be in need of. Furthermore, with a successful DC 15 Intelligence (Investigation) check, a member of the party can locate three flasks of Alchemist's Fire and spellcasting components up to 200 GP in collective value.

If the party searches the boxes, the noise disturbs 1d4 Swarm of bats which have been nesting in this room, which attack the players

The second level consists of an altar, atop which rests a dark, glowing crystal. The crystal fills the room with necromantic energy, which every player can feel makes them sick to their stomach. A player which uses the Detect Evil and Good spell can determine this room to be desecrated. A player which casts the Identify spell, targeting the crystal, or which succeeds on a DC 17 Intelligence (Arcana) ability check can determine that the crystal is responsible for animating the skeletons in Areas S3 and S4.

The energy pouring out from the crystal can be staved off in a few ways. Firstly, if the players can determine the command word, speaking it will deactivate the crystal. The command word is written in the necromancer’s spellbook, which has gone missing (see Area S4c for more details). Alternatively, a player casting Speak with Dead on the necromancer’s body (also in Area S4c) could determine it from asking him directly. A second option is to target the crystal with the Dispel Magic spell. For this purpose, treat the effect of the crystal as if it were a 5th level spell. Successfully using this spell on the crystal suppresses its effect for 1 in-game hour. Finally, if the crystal is somehow removed from this plane of existence, its effect ceases to spread throughout the stomach. If any of these actions (or any other creative means suggested by the players) are taken to deactivate the crystal, the twelve Skeletons and three Minotaur Skeletons located across Areas S3 and S4 are destroyed.

The room also features a set of stairs which lead further downwards

At the base of the tower is what appears to be a large study. In it is a large ornate rug, a desk with various books and magical trinkets spread across, and the door which leads out of the tower. Resting at the end of the room just opposite of the door is an empty pedestal, with the corpse of a humanoid figure in tattered robes clutching to it.

A cursory glance across this room makes it obvious to the players that foul arcane studies were conducted here. The corpse belongs to the necromancer, to whom the tower belonged. Afraid that he may one day be turned in the same way as the skeletons he commanded, he had inscribed various runes and wards into his skin to prevent his body from being animated. This function can be determined by a player examining the body with a DC 15 Intelligence (Arcana) check. A successful DC 12 Wisdom (Medicine) check reveals that the necromancer’s chest was ripped open by some monstrous force, but likely not from any of the skeletons. Any player who takes interest in the body also finds a small note in the necromancer’s robe containing the command word to open the tower’s door. 

A Way Out?

Any creature investigating the pedestal notices that the outline from dust indicates that a book once rested here, one arguably of great importance to its owner. A successful DC 15 Wisdom (Perception) or Intelligence (Investigation) check by anyone searching the room reveals that an arcane circle is hidden under the ornate rug. The use of the Detect Magic spell or a successful DC 12 Intelligence (Arcana) check reveals it to be a teleportation circle. The necromancer had bought a spell gem from Blingdenstone and had stored a casting of Teleportation Circle within it. This gem is embedded in the center of the circle, and can be successfully identified using the Identify spell. The gems function can also be deduced by a successful DC 15 Intelligence (Arcana) check.

Activating the gem would allow the players to use the Teleportation Circle to transport themselves to a location outside of the Dire Worm’s belly. If they know of another circle, they can choose it as the target destination. Otherwise, they are transported to wherever the necromancer had established another teleportation circle of the same type.

There is an obstacle preventing escape, however. The teleportation circle is missing a few of the necessary runes to function. If the players have discovered both the teleportation circle and investigated the pedestal, it becomes clear to them that to complete the circle they will need to locate the necromancer’s spellbook.

Unbeknownst to the party, the creature which murdered the necromancer stole his spellbook and has already fled beyond the Dire Worm’s stomach further into its body. 

The lower-right stomach of the Dire Worm houses the remains of a storehouse belonging to a dwarven mining crew. Found within it are various tools and equipment. Inspection of the equipment reveals that it is all degraded, and likely was so before the storehouse was swallowed. The storehouse was abandoned long before the worm swallowed it.

Among the old equipment is a keg of dwarven blasting jelly. Inspection reveals the jelly to be in good condition, but the fuse is heavily water damaged and cannot function. The party would need to either replace the fuse, or ignite the barrel by some other means - such as through use of the [Tooltip Not Found] spell or a vial of Alchemist's Fire (such as those found in Area S4a). If ignited, each creature within 10 feet of the keg must make a DC 12 Dexterity saving throw when it goes off (which is immediately if it is ignited by a means other than a fuse). On a failed save a target takes 8d6 fire damage, or half as much on a successful one.

A successful DC 12 Intelligence (Nature) or Wisdom (Medicine) check would provide the insight that if this keg were detonated within the belly of the Dire Worm, it may provide enough disturbance to the stability of the stomach to cause the monster to regurgitate its prey. Regardless of if they are aware, if the party detonates the keg within one of the four stomachs and the amount of fire damage dealt by the keg is 40 or higher, the Dire Worm spits up the party. If the damage dealt is not sufficient to cause regurgitation, the explosion attracts the attention of all monsters inside the stomachs, who converge on its location after 1d4 rounds.

Regurgitation

Being spit back up by the Dire Worm is not a comfortable path. The party will be surrounded by ejected stomach acid and debris. As it occurs, have each member of the party make a DC 15 Dexterity saving throw. A target takes 2d8 bludgeoning damage plus 3d6 acid damage on a failed save, or half as much on a successful one. If a creature fails this saving throw by 5 or more, it takes an additional 1d8 bludgeoning damage and 3d6 acid damage.

It is assumed that the monsters within the belly either find a way to remain in the stomach by clinging to its walls (such as the oozes) or are ground up in the process of being regurgitated. Once spit up, the Dire Worm does not take interest in the party and continues on its endless rampage unless the party attacks it.