This week’s encounter is Tunnel Terrors, a standalone one-shot that you can drop into any D&D campaign while the characters are exploring an underground region. From dwarven mines to an abandoned cave to an Underdark tunnel, this encounter can be placed anywhere underground. This encounter can be scaled to any tier of D&D play because of its simple premise: while exploring, the party is attacked and the tunnel starts to cave in!

Scaling this Encounter

This encounter can be scaled to suit characters of any level. Each version of this encounter is suitable for each of the four tiers of D&D play—optimized for play at 2nd, 5th, 11th, and 17th level. This is just the beginning, though. You can use the D&D Beyond Encounter Builder to further tailor this encounter to suit your party’s size and strength. If you’re playing with parties that are at the strong end of their tier, err on the side of increasing the numbers of the creatures in that tier, rather than pitting them against a higher-tier encounter.

For example, if you have a party of four 4th-level characters, you’re more likely to have a balanced fight if you increase the number of enemies in the tier 1 encounter. It might make more sense to reduce the number of monsters in a tier 2 encounter, but it’s easier to add power to a weaker encounter than remove power from a stronger one.

Why is tier 1 optimized for 2nd level, and not the lowest level of the tier (that is, 1st level) like the other tiers? Level 1 is a great starting point for new D&D players, but raising the base level of these encounters to 2nd level creates more opportunity to incorporate a diverse array of monsters and traps, rather than relying on the usual suspects for 1st-level adventures.

Combat Encounter: Tunnel Terrors

This encounter is suitable for characters of any level, if you follow the scaling guidelines.

The characters are traveling through a pitch-dark tunnel when the walls begin to tremble. Deadly creatures spill through the earth or stone in a cloud of debris. Worse still, the tunnel itself starts to collapse! The characters have to brace the walls and defend against the monsters’ onslaught if they want to survive!

Tier 1 Encounter: Ankheg Ambush

Read or paraphrase the following:

You trudge through a dark tunnel, about fifteen feet wide, with a squat ten-foot-tall ceiling. The air is dry and chilly, and the dirt beneath your feet is loose and slippery like sand. Suddenly, the wall of the tunnel ten feet in front of you explodes in a hail of dust and debris! Sandy soil begins to rain from the ceiling. What do you do?

The explosion in the side of the tunnel was caused by an ankheg. It knew about this tunnel and concealed its nest just a few feet away so it could sense prey passing through the tunnel with its tremorsense. This ankheg surprises characters in the party with a passive Wisdom (Perception) score of 10 or lower, and leaps upon one of the characters and bites them. Once a character is grappled by its pincers, the ankheg drags them away into its nest and tries to kill and devour them unimpeded. The lair is a simple cavern immediately adjacent to the main tunnel, 30 feet long and 10 feet wide. If the ankheg is pursued, it stops grappling its prey (no action required) and uses its Acid Spray to attack both its dropped prey and as many of its pursuers flooding into its lair as possible.

Tunnel Collapse. Part of its trap is the loose, silty soil of the tunnel itself; the force of digging through the wall collapses a 30-foot-long stretch of the tunnel. Then, once the tunnel has collapsed, potentially trapping other prey, the ankheg burrows out the tunnel and lays its cunning trap anew.

Each turn, silt and dry soil rains from the ceiling. On initiative count 20 (losing initiative ties) each creature in the tunnel must succeed on a DC 10 Strength saving throw or be knocked prone. Creatures inside the ankheg’s lair are considered outside of the tunnel. The tunnel collapses completely after 1 minute, and creatures without a burrow speed begin to suffocate.  

Tier 2 Encounter: Bulette Barrage

Read or paraphrase the following:

You travel through an underground cavern, about thirty feet wide with a low ceiling, only about ten feet high. Thick oaken beams stand like majestic pillars in the center of this tunnel, supporting the roof. Out of nowhere, the ground twenty feet ahead of you explodes in a shower of stone and wet earth! A gigantic creature, like a shark burrowing out of the earth, hurtles directly at the character in the front of your party, smashing through beams as it flies through the air!

This bulette surprises characters in the party with a passive Wisdom (Perception) score of 15 or lower. It bursts from the ground 20 feet in front of the characters, and uses its Standing Leap trait to jump straight at the character or characters in the front of the party’s marching order. It then lands on top of as many of them as possible using its Deadly Leap action. Once it does so, it burrows up to 20 feet into the wall of the collapsing tunnel using its remaining movement, preparing to hide before it attacks again.  

Tunnel Collapse. The tunnel began collapsing when the bulette smashed through several of the tunnel’s wooden support beams. These beams are placed at 10-foot intervals down the tunnel, at the bulette smashed through two of them in its leap at the characters. Unless the characters can find a way to replace both beams, the tunnel will collapse.

Wet earth and heavy stones rain from the ceiling. On initiative count 20 (losing initiative ties) each creature in the tunnel must make DC 13 Dexterity saving throw, taking 7 (2d6) bludgeoning damage and be knocked prone on a failed save. A creature that succeeds on its save takes half damage and isn’t knocked prone. Creatures inside the tunnels burrowed by the bulette are considered outside of the collapsing tunnel. The tunnel collapses completely after 1 minute, and creatures without a burrow speed begin to suffocate.  

Tier 3 Encounter: Behir Battle

Read or paraphrase the following:

You journey through a stone cavern, dimly bathed in teal light by faintly phosphorescent mushrooms growing along the walls. The tunnel is broad, easily forty feet wide, and the vaulted ceiling is equally as tall, though dozens of huge stalactites dangle like daggers from above. Suddenly, a bolt of lightning bursts from the top of the cave, crashing into the front rank of the party!

A behir clings to one of the one of the many stalactites hanging from the top of the cavern. While it waits for its Lightning Breath to recharge, it constricts the stalactite it is coiled around (AC 17, hp 18), and leaps to another. Every 5-foot square of this tunnel’s ceiling contains a stalactite large enough for the behir to hang from.

Stalactite Collapse. If the behir reduces the stalactite’s hit points to 0, the stony spear falls from the ceiling and crashes down to the ground below. A creature in the stalactite’s space must make a DC 18 Dexterity saving throw, taking 17 (3d10) piercing damage plus 14 (4d6) bludgeoning damage and being knocked prone on a failed save, or half as much damage on a successful save. Additionally, all creatures within 10 feet of the stalactite when it strikes the ground must make a DC 18 Strength saving throw or take 14 (4d6) bludgeoning damage and be knocked prone as they are bombarded by stone and debris.

Developing Tactics. Once the behir is reduced to half its hit point maximum, it stops collapsing stalactites and starts using them to hide. It uses its action to Hide on each of its turns, and then sneaks along the ceiling and walls until it’s behind the characters. It then either uses its Lightning Breath to blast as many of them as possible, and then flees if at least two of them are still conscious. Alternatively, if only one creature is near the behir, it grapples them using its Constrict attack, bites it, and then flees away from the party down the tunnel.

Tier 4 Encounter: Purple Worm Power

Read or paraphrase the following:

You travel through a vast underground chasm. The ceiling is so high you can’t see the top, and the tunnel is easily one hundred feet wide. Strange green flames burn high up on the rocky crags of the tunnel’s wall in the distance, like campfires of unknown creatures that peer down into the darkness. Suddenly, the ground starts to tremble, then quake, and then explodes in a shower of granite shards as two enormous worms burst from the floor, just ten feet in front of you. The tremors in the earth continue, and massive boulders tumble down the walls.

This mated pair of purple worms hunts together, and stumbled upon the characters as they burrowed their way through the earth. The worms attack, thinking the adventurers will make a quick snack. A worm flees if its mate is killed or if it is reduced to one-quarter its maximum hit points.

Combat begins as a cloud of debris explodes in front of the worms, which dissipates after 1 round on initiative count 20 (losing initiative ties). They are heavily obscured to creatures without blindsight or truesight on the other side of the dust. A worm begins combat by lashing out of the cloud of debris with its tail stinger. Since the attack is so sudden, it has advantage on any creature that couldn’t see the heavily obscured worm. The worm then follows up its stinger attack by lunging forward to bite a character. If it is able to swallow the character, it begins burrowing straight down, ready to escape with its snack and move on.

Falling Boulders. Enormous boulders tumble down the walls of the cavern, shaken free by the rumbling passage of the purple worms. Each round on initiative count 20 (losing initiative ties) a boulder falls down the wall and rolls through the passage in a line that is 100 feet long and 15 feet wide. Any creature in this line must make a Dexterity saving throw, taking 44 (8d10) bludgeoning damage and being knocked prone on a failed save, or takes half as much damage and is not knocked prone on a successful one. The DM determines the direction and angle of this line. It can also be determined randomly, using a d12 to choose its origin, arranging 1 through 12 like the hours on a clock face; the boulder then rolls towards the center of the cavern. The tremors stop and the boulders cease to fall after 1 minute.

Fleeing Worms. A worm that has begun escaping lashes out with its stinger as it flees, and will even stop to consume another creature that pursues it. It won’t escape without a snack (unless it’s already fleeing in fear), and will stop its flight and try to swallow at least one other creature before resuming its escape.

Treasure

Consult the random treasure tables in chapter 7 of the Dungeon Master’s Guide to determine the amount of treasure these creatures are carrying. Choose the Individual Treasure table that suits the challenge rating the encountered creatures. Then roll once for each creature that would logically carry treasure on its person.

Use your best judgment, and always try to reward the characters at least a little bit for their success. For example, the ankheg the characters defeat in the tier 1 encounter logically wouldn't carry any treasure with it, but its lair might contain the body of several dead travelers that it's killed over the months its been here. Roll on the Individual Treasure: Challenge 0–4 table once.

Did you like this encounter? Check out the encounters in the Encounter of the Week series. You can also pick up the adventures I've written on the DMs Guild, such as The Temple of Shattered Minds, a suspenseful eldritch mystery with a mind flayer villain. My most recent adventures are included in the Platinum Bestseller Encounters in Avernus, a collection of over 60 unique encounters created by the Guild Adepts, which can be used to enhance your campaign in Avernus or elsewhere in the Nine Hells. Also check out the Platinum Bestseller Tactical Maps: Adventure Atlas, a collection of 88 unique encounters created by the Guild Adepts, which can be paired with the beautiful poster battlemaps in Tactical Maps Reincarnated.

James Haeck is the lead writer for D&D Beyond, the co-author of Waterdeep: Dragon Heist, Baldur's Gate: Descent into Avernus, and the Critical Role Explorer's Guide to Wildemount, a member of the Guild Adepts, and a freelance writer for Wizards of the Coast, the D&D Adventurers League, and other RPG companies. He lives in Seattle, Washington with his fiancée Hannah and their animal companions Mei and Marzipan. You can find him wasting time on Twitter at @jamesjhaeck.

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