Kobold tunnel traps

Pit Trap

DC Ability Action

10* Wis (Per) to spot 

14* Int/Dex to disarm or bypass

12* Str (Ath) to avoid falling in and taking damage

Dex (Acr)

*Checks made with disadvantage when running or in darkness

Effect: 5’ fall into a pit for 1d4 damage

These traps are simple in execution, and easily avoiding by anyone proceeding cautiously. They are only dug about 5 feet deep, the width of the tunnel (5-10’), causing d4 damage when fallen into. The kobolds don’t have a great sense of their small size, and dug pits that work well against beastly intruders, and sneak thieves. They can be easily bypassed by walking a narrow lip along either wall, or hopped across (5’ gap). In the dim light of the tunnels, they can be difficult to notice and cause problems.

Pendulum Trap

DC Ability Action

16* Wis (Per) to notice tripwire

20* Wis (Per) to notice pendulum above

20* Dex (Thieves’ Tools) to disarm

10 Str (Ath) to step over the tripwire

Dex (Acr)

*Checks made with disadvantage when running or in darkness

Effect: 3d4 damage to anyone in the arc of the pendulum

The kobolds have suspended logs on a rope to the ceiling of the tunnel. When the tripwire is triggered, the holding mechanism is released, and the log swings down through a 15’ long arc, centered on the trigger point, slamming into anyone in its path.

Pit and the Pendulum Trap

DC Ability Action

10* Wis (Per) to spot trap

14* Int/Dex to disarm or bypass trap

12* Str (Ath) to avoid falling in and taking damage

Dex (Acr)

20* Wis (Per) to notice pendulum above

20* Dex (Thieves’ Tools) to disarm

*Checks made with disadvantage when running or in darkness

Effect: Falling into the pit triggers the pendulum, which sweeps down from both sides of the ceiling above, dealing bludgeoning damage, and forcing a Dex ST to avoid being knocked into the pit for additional damage. Falling into the pit causes 1d4 falling damage, and the pendulum causes 3d4 bludgeoning damage.

The pit looks like any other kobold pit trap, but there are two logs suspended on ropes to the ceiling on either side of the pit. There is no tripwire to notice for the pendulum trap, as it is set off by a pressure plate when someone falls into the pit. 

Arrow Trap

DC Ability Action

14* Wis (Per) to notice

16* Int/Dex to identify trigger plate in the floor

16* Dex to avoid being hit by arrows

*Checks made with disadvantage when running or in darkness.

Effect: 1d6 arrow damage per 5’ affected area

These are more intricate traps, and more difficult to notice. Again, the traps are meant more for wandering beasts and thieves, so they only deal minimal damage, 1d6 per affected square. Usually, these are set in straighter sections of tunnel, with a 15’ run of tunnel trapped with arrows, and a trigger plate or tripwire at one end of the setup. There is a 1 in 6 chance that they party triggers the near end of the trap, and it goes off without harming anyone. Otherwise, anyone in the affected 15’ area must save or take 1d6 damage from arrows.

Variant: Poisoned Arrows

As above, with an additional 1d4 poison damage on a failed save to avoid.

Rolling Logs Trap

DC Ability Action

15 Wis (Per) to notice it is not a real gate

Int (Inv)

17 Dex (Thieves’ Tools) to disarm the release mechanism

17 Dex to avoid the collapsing wall of logs

18* Str to move the wall of logs aside once disabled

*Checks made with advantage if more than one work together

Effect: 2d6 damage within 10’ of the wall

The only way forward is blocked by a rough gate of logs. On one side of the tunnel, there is a hasp mechanism that can be opened. It resists opening, and will need to be forced with a Strength check. This causes a wire to be pulled, releasing the stack of logs, which tumble forward 10’ into the tunnel. Anyone in that area must save or take the listed damage.

Oil Trap

DC Ability Action

15* Wis (Per) to discern the smell of oil

18* Wis (Per) to notice the small spouts above the torches set in either wall

16* Wis (Per) to notice the tripwire across the floor just beyond the torches

18 Dex to avoid the direct splash of oil

*Checks made with disadvantage when running

Effect: 3d4 fire damage from burning oil, half damage on a successful save, all within 5’ of the trigger point

The tripwire is well concealed, and difficult to notice. There are three small spouts above each torch, one set into each side of the tunnel. The tripwire causes oil to flow out of the spouts, igniting as it arches over the flames, and splashes onto anyone in the trigger square, or 5’ before or after.

Volcanic Lair

At the heart of the volcano is the lair of the red dragon, Antarexes Glavenis. There is a large central cavern where the dragon rests. It has a large flat area in the center of the cavern where the dragon has made its nest. This is where it keeps its treasure hoard, and where it perches in the lair.

The floor of the cavern is scarred with lava pools, and the walls of the cavern are riddled with tunnels. There are shelves jutting out from the cavern walls, mostly around the openings to the tunnels, and at their height. The cavern is 200 feet from floor to ceiling, with a large central chimney opening to the outside air beyond, and with ledges and tunnels at 80 feet off the floor. The dragon's perch is 100 feet above the cavern floor.

The walls of the cavern are pocked with tiny holes, allowing anyone to climb the walls with a successful DC15 Athletics check. If they use ropes or other climbing aids, they can make the check with a DC10. Success allows characters to climb at their climbing speed (default is half speed). There are even tunnels in the dragon's perch, but none connecting to the outer walls.

If the party is patient and fortunate, they can find themselves in the enormous cavern for several hours while Antarexes is out of the lair, flying over the countryside, searching for the threat it feels is approaching. While the dragon is away, the party has the opportunity to explore the cavern and choose where to be when it returns. They can set themselves up to attack it without openly exposing themselves.

If the dragon is present when the party arrives, it will sense their presence. If they remain out of sight in the tunnels, it will just get restless, fly around the chamber and settle, then out the opening at the top of the chamber. It will circle the mountain several times, then head off to scour the countryside for food or evidence of intruders. If the party don't take actions to conceal themselves, then the dragon will feign sleep and wait for them to expose themselves.

Dragon's Lair

There is so much more to add in order to call this adventure finished, but I feel like I need to touch on all points at least, before worrying about filling in the missing details.
After escaping from the kobolds, the party make their way through a fiery subterranean world toward the dragon's lair. This is where the aspects of dragon's lair actions really manifest themselves. One was to have fire elementals appear. This will keep the party sharp on their approach. I don't think they should be overwhelmed at this point, but fighting fire-based creatures can hone their tactics before confronting the dragon itself.
There is the added benefit of draining their spells and other resources, forcing them to pause for a rest prior to approaching the dragon itself. I like this idea, it allows them to rethink spell choices and so on, as well as watch the movements of the dragon in order to better face it. Given that this quest has been a dragon quest, and the final confrontation is with the dragon itself, I feel better about having them fully prepared before engaging it. If they are at their best and strongest, they will present a better challenge to the dragon, and that is what is wanted.
When the party entered the kobold warrens, the dragon was in flight to destroy the valley around the dragon cult's base. By the time they have escaped and found the lair itself, the dragon will have returned. The route from the kobold warrens to the main lair should give a glimpse into the inner sanctum from a safe location. I don't want it to reveal the full layout of the lair, just be enough to show the presence and movement of the dragon. The dragon is still restless, and will take flight each night to scour the countryside in search of attackers.
This allows the characters an opportunity to quickly search the dragon's main chamber and plan their encounter. They may find spots they can use to trigger an ambush, and with proper spells, they could potentially set some traps and snares of their own.
Again, this all needs to be mapped and plotted, but at least this begins the process.

Into the Kobold Den

Den is an operative word. The underground tunnels are narrow and winding, a warren of darkness and decay. Foul smells permeate the air. Very quickly it becomes apparent that the tunnels are also a maze, winding through and across themselves as they curl through the mountain's base.

The party should realize that their best hope lies in secrecy and stealth. This section is meant to test their ability to remain out of sight and move quietly. The exact layout is less critical, and the numbers of the kobolds are meant to be overwhelming. Having them meet a group of 5-8 or 8-10 should convince them they are better off avoiding open conflict if possible.

I think a few small skirmishes are fine, but as they move through the tunnels, the kobolds should increasingly try to fall back and signal a general alarm. It is necessary to prevent them from escaping to do so. Once they do, have them start arriving in ever larger groups. Eventually, they will overwhelm them and take them prisoner. They will tie them all up and transport them to a bone-barred cell in a large central chamber.

The party awake to a large bonfire burning in the center of the chamber. Smaller fires burn around the edges of the chamber. There are large numbers of kobolds singing and chanting as they dance around the fires, casting eerie shadows on the walls and ceiling of the cavern. There is a raised platform on one side of the large fire, and the leaders of the clan sit there feasting.

The festivities last long into the night, and through dawn of the next day, although no sign of daylight reaches the cavern. Anyone understanding kobold will realize that they believe they have done a great service to their red dragon master, and are celebrating their deed. The following evening, they plan to march the party through the caves to the dragon's lair, and turn them over as an offering.

They have failed to disarm or disable the party in any way other than to lock them in a cage. If the characters have sense enough to rest and keep their cool, they will find themselves mostly alone in the middle of the day, with a small number of kobolds (around 20) lying around the large common area asleep or passed out. They can escape the cage with a DC16 strength check, or by picking the lock DC14.

Once free, they will need to escape the cavern and tunnels. With a survival check DC18, they will have been able to determine the direction of the dragon's lair and take their leave in that direction.

I will need to work up some stealth numbers, directional checks, and add some traps, since I feel many of the side tunnels would be riddled with pit traps, net traps, and dart traps. None of them is particularly damaging, since they were designed by kobolds, with little understanding of the larger world and its strength.

Back to the Kobolds

After dealing with the dragon cult, the party can make their way back toward the crossroads and the trail leading to the kobold den beneath the volcano. They can get away from any stragglers without worry. The copper dragons will swoop down to eliminate any survivors that try to follow them. After the party is gone and back on their own journey, the copper dragons will descend into the valley to eradicate any remaining cultists. They will collapse the tunnels and destroy the fortress, avenging their betrayal and displaying the full extent of their rage with the cultists. Terrible noises will chase the characters on their way back south toward the dragon lair.
Unfortunately, this will also awaken the dragon to trouble stirring in the valley. As the party regains the area near the crossroads, they will be aware of an approaching patrol from the direction of the kobold lair. They can take shelter below the edge of the road among some low scrub trees, successfully staying out of sight with Stealth checks of DC10 or better. As long as half or more succeed, they kobolds pay no heed to the party.
If the party remains hidden, they overhear snippets of talk about "the restless dragon... at least we'll get a break... gone to deal with it, whatever is gone wrong... seems worried about something, given the increased patrols."
They barely pause as they continue past the intersection, down toward the swamp. After a few tense minutes, you hear nothing, and emerge to continue. They can proceed as normal, with a DC15 Perception check alerting them to one or two more patrols as they go, requiring a Stealth DC12 to get off the road and hide, again with half the party succeeding being enough to escape detection.
Before long, a terrible roar will part the night, as a wave of heat passes overhead toward the north. The party freeze in their place, realizing the dragon has just passed over, but it passes quickly given the talk they've overheard from the kobolds.
They can press on more quickly now, and if they choose, can get within a few miles of the kobold lair by morning light. They can find shelter away from the road before dawn, and though without a campfire, they can feel safe throughout much of the day.