Here is an adventure for you to take advantage of.  i am not going to give any back story to this dungeon as it is something then that you can use in your own game in the way that you want.  Below is the map for the dungeon and each room is described after that.  My players have finished this dungeon and are moving on so I hope some of you get to use it too.  The adventure is suited to a group of four to 5 4th level adventurers, with the expectation that they will come out the other end as 6th level adventurers.

Free 5E Adventure!

All of the rooms in the dungeon are 20′ in height unless stated otherwise.  Any room with a split level (e.g. room 5) has 45′ of height.  All secret doors are a Perception DC 15 to detect an anomaly and a DC 20 Investigation check to activate.  The adventure starts with the players entering at the stairwell marked with an asterisk.  When I used this dungeon the group were transported their by means of a trapped Glyph of Warding and the stairwell lead to a blank wall.  In other words, they needed to find their way to the planar portal room and exit through the portal to return to their home world.  I also skinned the dungeon to appear as if the group were inside some massive, disgusting beast with bones, fleshy walls and grotesque slime everywhere.  This freaked them out a little but do with it as you will!

Room 1

This small bare room holds little more than a table and some chairs for the guards to rest at whilst waiting for new arrivals.  4 Guards (p.347 MM): 1 will attempt to alert the barracks if a fight breaks out.  Treasure: 17 silver pieces and a 4 leaf clover pressed in a book entitled Ettin’s Ettiquette, the do’s and dont’s of giant society.

Room 2

This room is set up like a large classroom with a slate board at one end with diagrams of a spider’s anatomy drawn on it, many desks facing the slate board and a large stone table that currently has a dissected phase spider laying on it.  Beside the stone table there is a large object covered with a thick velvet drape.  Underneath the drape is a glass cage and a living phase spider.  The phase spider will phase out when the drape is lifted and attack.  Inside a locked cupboard there is the instructors valuables (DC 10 thieves picks to open).   The cupboard holds 2500 CP, 1300 SP, 60 GP, 12 pieces of onyx in a silk pouch (50 GP each), Slippers of Spider climbing, Battle Axe +1, Boots of the Winterlands, Bracers of Archery.

Room 3

A steady drone of chanting comes from this room as the cultists within attempt to summon a creature from the abyss.  This large open chamber contains a large purple glowing symbol on the floor that pulsates with the chanting.  In the center of the symbol is a sacrificial chamber with a young female child strapped to it.  Standing above her with a sacrificial dagger is a cultist fanatic and around the symbol are 8 cultists (p.345 MM).  Bookshelves on the walls surround the room show arcane titles.  Any interruption will result in the cultists attempting to capture the adventurers for later sacrifice.  Treasure: Selected arcane texts on the shelves can be taken that value at 1000 GP to the right collector.

Room 4

This room is obviously a kitchen.  The ovens are still warm and the preparation areas are very messy, obviously after a meal service.  No one is currently in the room.  One of the cooks has his knife (a +1 magical dagger) and his fortifying drinks (two healing potions) hidden up the chimney near an oven.  DC 15 investigation check to find them.

Room 5

This room contains a dual level dining room.  The lower room has cheap wooden benches that are currently littered with plates and remnants of food, whilst the upper, overlooking chamber has fine tables, cutlery and refined silverware.  The upper section appears to be open but there is actually a permanent wall of force that separates the two rooms.  In the upper chamber there are 8 commoners clearing the dishes that may spot the adventurers as they enter the lower area, and definitely will if they enter the upper area.  The commoners are captives in the area and include the cook whose magical items appear in room 4.

Room 6

The western door to this guard room is trapped and if it is not opened while saying the password “Fantima” a Glyph of Warding throws a flame westward 30 feet (DC 11 Dexterity  1d10 damage, half on a successful save) DC 15 spot check to find.  10 Guards are in various states of readiness inside the barracks (p.347 MM).  No valuables (apart from their weapons and armor are kept here because there has been a spate of thievery in the barracks of late.

Room 7

This room has a split level, the lower half is littered with tables, ink stains and discarded parchment with nothing of value on it.  Above on a balcony a veteran guard stands (named Belell p. 350 MM) who controls a ladder that he can drop to approved visitors and priests.  Treasure: 12 SP in a coin pouch on Belell.

Room 8

Houses the 6 priests of this cult though only two are occupying this room currently (p.348 MM).  They have learnt Inflict Wounds instead of Cure Wounds in their spell list.  Names are Trillian and Baxter. Treasure: 1200 CP, 600 SP, 40 GP, 11 Eye Agate in a leather bag (10 GP each), Potion of Thunder Resistance

Room 9

There are a bunch of mops and brooms in this closet.

Room 10

This room contains a well in the middle.  The floor of the room is made up of a rusty iron grate below which is a 5 inch deep pool of water that reaches out to the edges of the room.  This conceals a water weird (MM) that has been tasked to attack any person that enters this room that does not wear the symbol of the priests God/patron.  This room is otherwise used to collect drinking water, a latrine and a washing area.

Room 11

This room is open to the corridor and is exceptionally cluttered with all sorts of apparatus and samples of tissue and organs from planar creatures in jars.  There is a good deal of crystal alchemy equipment, most of which is in poor disrepair.  Hidden under one of the workbenches is a small trapped chest (DC 10 spot to find).  Trap requires a DC15 Investigate check to find and a DC 15 Thieves Tools check to disable.  If opened without disabling a dart shoots from the trap at the person opening gaining a +8 attack to do 2(1d4) damage and a successful Constitution (DC 15) check must be made or they take a further 5 (1d10) from poison and gain the poisoned condition for 1 hour.  Inside the chest is 1800 CP, 1500 SP, 40 GP, 3 Crystal Decanters worth 25 GP each.

Room 12

Inside this room there is a large chest on an elevated stone plinth 5′ in height.  The plinth is 10′ by 10′ and the chest is unreachable simply by stretching to touch it.  If an adventurer climbs the plinth the doors to the room shut and lock and a poisonous gas fills the chamber.  Trap requires a DC 15 Investigate check on the plinth to be discovered and a DC 10 Thieves Tools test to disable.  The chest is unlocked and contains only 4 Carnelions at the bottom of it worth 25 GP each.  The poison gas does 1d6 per minute that it is present with no save.  It dissipates when a door is opened.

Room 13

This used to be a prison but is filled to around 15′ with water that has seeped into the room from a cave in.  Inhabiting the room now is a giant constrictor snake (p.342 MM) that the priests use to take care of captives that get out of hand.  4 animated skeletons (p.272 MM) are also located here, a remnant from the days that this area was still a functional prison.

Natural Chimney 14

The complex makes use of a natural chimney at this point to separate the lower chamber from the planar portal section.  It is a smooth wall that rises 20′ to the next level requiring a DC 10 athletics check to climb.

Room 15

This room houses the guards of the portal area, a pair of spined devils who are likely to have heard the approach of the players unless they were exceptionally stealthy coming up the chimney.  They hide by clinging to the walls awaiting the players to enter the chamber.  The spined devils have been bargained with by the priests to provide protection until they have negotiated with their master and they place all items they capture from infiltrators in an unlocked chest here.  If the spined devils are looking like they will be beaten they will attempt to alert the other guards in the barracks at room 16.  Spined devils are on p.78 of the MM.  Treasure: 1800 CP, 600 SP, 50 GP, 5 Carnelions (5 GP each), Potion of Hill Giant Strength, Ring of Swimming, Potion of Poison, Potion of Lightning Resistance.

Room 16

This room is the barracks for the off duty spined devils.  It is a cesspit of depravity with body parts, gruesome collections of flesh formed into strange sculptures and arcane symbols best left unread.  3 spined devils rest here waiting for their next shift.  There are 5 other spined devils roaming this area that may be used as random encounters throughout the portal section.  No treasure is to be found here.

Room 17

This is a temple room for the priests (p.348 MM).  Two of them always tend the room but they are currently down the corridor to the south feeding the Oni through the pit trap.  From the layout and accouterments of the room it should be abundantly clear that this group of priests worship a devil from the nine hells.  Regardless of this, the items in this room are valuable and worth 2000 GP to the right collectors.  Looting the room will take 10 rounds to do.  On each round of looting roll 1d20 – on a roll of 17+ the priests from the south hear the noise and return to battle the adventurers desecrating their temple.

Pit Trap at 18

DC 10 to spot and DC 10 with thieves picks to disable.  Players must be actively looking for it though.  If trap is sprung it requires a DC 10 Dexterity save or fall 30′ to the planar portal room below (Room 22).

Room 19

Lavishly appointed visitor chambers that are assigned to the planar envoys that the head priest (who is away with one of the other priests securing another item to bargain with) deals with in making a deal with the devil beyond the planar portal.  The room is currently occupied by a barbed devil (p.70, MM) who is waiting to see the head priest and consider his latest offering.

Chimney at 20

This chimney is 90′ from top to bottom with a landing at the top that leads to the passage to Room 21, a passage halfway where the corridor meets it from the main complex and a landing at the bottom that leads to the secret door into Room 22.  Otherwise the chimney is the same as the one at 14.

Room 21

A large room that has at its centre a crystal prism, encased in the prism is the Axe of the Sun, an artifact of the sun God Apollo (or whoever the sun god is in your game).  The prism is easily opened and the axe is one of the intended gifts for the devil that the head priest is dealing with.  He has not left it unguarded though.  A Gorgon protects the weapon from any intruder bar the high priest.

Axe of the Sun

Intelligence: 13  Wisdom: 12  Charisma: 12  Alignment: Chaotic Good

• Communicates using emotional empathy
• has hearing and darkvision out to 120′
• Seeks to defend the followers of Apollo
• +1 normally but in full sunlight is a +3 weapon of warning
• Is distracted by beauty
• Honesty is of the utmost importance to it
• Beauty and independence are its ideals
• Has a sister weapon (Scimitar of the Moon) that it is insanely jealous of

Room 22

A large portal stands at the center of this large chamber.  It can be used to connect to several locations as the DM so desires.  It is guarded by an Oni who seeks not to kill but to capture.  If it captures all of a party it randomly selects a location, strips the captives of their gear and throws them through the portal, keeping their items for itself.  The Oni has been promised a great service by the devils that use this portal for the protection of the room.

Room 23

This room is a crafting room and is filled with all sorts of crafting materials, a large table and chairs.  Much of the cultists handiwork can be found here including holy symbols of the devil that the priests follow along with some whittled statuettes of the same.

Hope you enjoy the adventure.  Keep rolling!