The Great Tunnel Collapse

Location: The Deep-Strands, Sub-Terran Accord
Level Range: 2-4

Background & Objective

The Deep-Strands—a network of indestructible Aether-glass tunnels—are the lifeblood of the Sub-Terran Accord. During a high-stakes summit between the kobold engineers and delegates from the Steam Wrights Consortium, a localized tectonic shift caused a catastrophic structural failure in the "Vein of Valerius."

As recovery crews led by Grand Warren-Master Zik-Zik began excavation, they inadvertently pierced a dormant pocket of raw Aether. This exposure triggered a rapid Aether-crystal bloom, "fusing" the rubble into a volatile, crystalline lattice. The diplomats are trapped in a pocket where the air is thinning and the very walls are becoming magically over-saturated. The party must clear the debris, harvest the volatile crystals to prevent a "Mana-Burst," and extract the survivors before the tunnel becomes a tomb.

Sight, Sound, and Smell

The air is a thick, shimmering haze of cobalt dust that tastes like copper and ozone. The rhythmic, frantic ticking of clockwork rescue-drones echoes against the jagged, glowing protrusions of Aether-crystals that pulse with a low, rhythmic thrum. Beneath the hiss of escaping steam from shattered pipes, there is the unsettling, wet sound of Mycelial threads knitting themselves into the rock.

The Gimmick: Limited Air & Aether Saturation

The encounter operates on a ticking clock measured in Breaths (Rounds).

The Oxygen Clock: The party has a total of 20 rounds before the air becomes unbreathable.
The Saturation Meter: Every time a player uses a spell of 1st level or higher, the Aether-crystals glow brighter. If 10 spells are cast within the tunnel, the crystals detonate in a World-Fire Spark, dealing 8d6 force damage to everyone in the area and collapsing the tunnel permanently.
Blue Bile Hazards: Puddles of caustic Blue Bile (the Mycelium’s waste product) have leaked from the bedrock. Stepping in it deals 1d6 acid damage and forces a DC 12 Constitution save to avoid a minor mutation (e.g., skin turning translucent blue for 1 hour).

Key NPCs

Grand Warren-Master Zik-Zik

The leader of the Accord's engineering corps, Zik-Zik is a frantic kobold wearing a brass-plated exoskeleton and a pair of cracked Aether-lenses.

"The stone does not hate us, but it is currently very, very heavy, and quite frankly, quite explosive! Pivot the gears! Pivot!"

Ideals: Structural integrity is the highest form of worship.
Flaws: Panics if his mechanical "Strand-Stalkers" are scratched.

Venda 'Sporelung'

A deep-delving merchant who happened to be nearby. She is a mutated humanoid who breathes through a specialized filtration mask filled with Blue Bile.

"I’ve seen the earth swallow better men than you, surface-dweller. Buy a filter or prepare to cough up your soul."

Ideals: Everything has a price, even survival.
Flaws: Obsessed with collecting Aether-dust, even in life-threatening situations.

Rumors & Whispers

True: The collapse was not an accident; it was a "Desperate Surgery" by the Omnisporangium to seal a nearby Void-leak that was poisoning the Deep-Strands.
False: The Obsidian Scale planted a mana-bomb in the tunnel to assassinate the Steam Wrights delegates.

Tactical Combat & Challenges

Clearing the Rubble

Players can spend an action to make a DC 14 Strength (Athletics) check to clear debris. Alternatively, a DC 15 Intelligence (Arcana) or Dexterity (Sleight of Hand) check allows a player to safely "harvest" an Aether-crystal, removing it from the lattice and reducing the Saturation Meter by 1.

The Bile-Mutated Cave-Crawlers

The scent of raw Aether has attracted Cave-Crawlers—multi-legged insectoids mutated by the Mycelium’s secretions.

Tactics

Cave-Crawlers use their Cling ability to attack from the ceiling, dropping onto players to drag them into puddles of Blue Bile. They focus on players who are actively harvesting crystals, sensing the shift in magical energy. They are skittish around open flames but are empowered by spells, gaining 5 temporary hit points whenever a spell is cast nearby.

Ecology

These crawlers are the "scavenger cells" of the planetary immune system. Normally, they break down inorganic waste, but when exposed to high-grade Aether, their carapaces harden into crystal, making them aggressive and territorial.

Markings & Signs

Near the entrance of the collapse, a series of jagged symbols are scratched into the copper plating of the tunnel wall.

The Marking: A circle with three dots inside, crossed by a jagged lightning bolt.

Translation: "Thin air, thick magic. Do not spark."

Narrative Outcomes & Loot

Success ensures the safety of the Sub-Terran Accord delegates, granting the party "Friend of the Deep-Strands" status. This provides a 20% discount at all Accord trading posts and access to the Deep-Strand transit system.

Featured Loot: The Accord Regulator

Wondrous Item, uncommon (requires attunement) This copper gauntlet is embedded with a stabilized Aether-crystal shard.

Lore & Origin

Originally designed by Grand Warren-Master Zik-Zik during the "First Bloom," these regulators were intended to allow kobold miners to work in high-magic zones without triggering Wild Magic surges. The gauntlet hums with a faint, reassuring vibration, a reminder of the Accord's mastery over the volatile subterranean forces of Aethervale.

Ability: While wearing this gauntlet, you have advantage on saving throws against environmental magical effects (such as Wild Magic zones or Blue Bile mutations). Additionally, you can cast Identify once per long rest, but only on crystals or mechanical objects.