The classic setting of Basic D&D invites exploration across a richly diverse world where Viking-inspired Northlands, Arabian Nights-themed emirates, and ancient Greco-Roman empires coexist alongside a hidden Hollow World within the planet itself. When an ancient Nithian artifact surfaces in a remote frontier town, the adventurers are drawn into a conspiracy that spans cultures, continents, and the boundary between the outer surface and the forgotten civilizations preserved inside the world.
Story Arcs
The Frontier Town of Threshold
The adventurers begin in Threshold, a frontier logging town in the Grand Duchy of Karameikos, where diverse cultures — native Traladarans and colonizing Thyatians — coexist uneasily. When a collapsed mine reveals an underground chamber filled with strange hieroglyphs and a golden scarab artifact, the party is hired to investigate. The ruins predate all known civilizations and the hieroglyphs match no living language. A scholar identifies them as Nithian — a civilization that the Immortals (Mystara's deific beings) erased from history itself.
Across the Known World
The Nithian scarab is a key — one of five needed to open a gateway to a place the fragmented texts call "the World Within." The other four keys are scattered across the Known World: one in the Northlands of Ostland, one in the magical Principalities of Glantri, one in the desert Emirate of Ylaruam, and one in the ancient Thyatian Empire. The adventurers must travel across cultures and continents, navigating political rivalries and local dangers, to assemble the keys before a secretive cult dedicated to the Nithian Immortal Thanatos collects them first.
The Hollow World
With all five keys assembled, the gateway opens and the adventurers descend into the Hollow World — a vast inner surface lit by a permanent red sun, where the Immortals have preserved civilizations that were destroyed on the outer surface. Living Nithians, Azcans, and other lost peoples thrive here, unaware that an outer world exists above them. The cult of Thanatos has already arrived and plans to use the Nithian keys to collapse the barrier between worlds, destroying both surfaces. The adventurers must rally the Hollow World nations and stop the cult before the world turns inside out.
Session Outlines
These outlines are for planning reference only. Actual sessions are not created by the template.
The Mines of Threshold
Introduce Threshold: a frontier town of wooden palisades and log buildings where Traladaran natives and Thyatian colonists mix uneasily. The party is hired by Baron Halaran to investigate a mine collapse that revealed ancient underground chambers. The chambers contain hieroglyphs depicting a civilization of pyramids and sun-worship, and a golden scarab that pulses with warm magical energy. The mine is now infested with giant centipedes and a basilisk that wandered in through the broken wall. After clearing the creatures and recovering the scarab, the party meets Scholar Claransa, who identifies the writing as Nithian — a word that causes her visible fear.
Secrets Erased from History
Claransa explains the terrifying truth: Nithia was a civilization so corrupt that the Immortals (Mystara's gods) didn't just destroy it — they erased all memory of it from every living mind and every written record. The fact that physical ruins still exist means the erasure is failing. The scarab is one of five keys to something called "the Threshold Between Worlds." That night, cultists in black robes attack Claransa's home, seeking the scarab. They wear the symbol of Thanatos, Immortal of Death and Entropy. After the fight, a dying cultist reveals the other keys' general locations.
The Northlands — Ostland
The party sails north to the Viking-inspired Northlands. Ostland is a land of fjords, longships, and warrior-culture. The second key is embedded in the prow of a legendary longship sunk during a battle with a sea serpent decades ago. The party must earn the trust of Jarl Sigurd by participating in trials of strength and courage (wrestling, sailing, drinking), then dive to the wreck site, battling underwater threats. The sunken longship is guarded by draugr — undead Vikings who still man their posts.
The Principalities of Glantri
Glantri is a magocracy where noble houses of wizards rule and clerical magic is outlawed on pain of death. The third key is locked in the vault of the Great School of Magic, catalogued as a "curiosity of unknown origin." The party must navigate the cutthroat academic politics of the school, where rival wizard princes use students as pawns. The party can steal the key, earn it through a magical challenge, or trade a favor to a Glantrian prince. The cult of Thanatos has agents among the students.
The Desert of Ylaruam
The Emirate of Ylaruam is an Arabian Nights-inspired desert kingdom ruled by the teachings of the philosopher al-Kalim. The fourth key lies in a buried Nithian temple beneath the shifting sands. The party must hire a desert guide, survive sandstorms and scorpion swarms, and navigate the temple's traps — designed to test worthiness through puzzles of Nithian mathematics and astronomy. A medusa has taken up residence in the temple's inner sanctum, turning previous explorers to stone.
The Thyatian Empire
Thyatis, modeled on ancient Rome, is a sprawling empire of gladiatorial arenas, senate politics, and military might. The fifth key is worn as a pendant by a Thyatian senator who has no idea of its significance — he inherited it as a family heirloom. The party must obtain it through political maneuvering, a gladiatorial challenge, or subterfuge. The cult of Thanatos makes a major play here, attacking the senator's villa with a strike team, forcing the party into a defensive battle in the heart of the imperial capital.
The Gateway Opens
With all five keys, the party returns to the underground chamber in Threshold. Claransa's research reveals the activation ritual: the five scarabs must be placed in specific positions during a particular celestial alignment — which is tonight. The cult of Thanatos arrives in force, triggering a battle in the underground chamber even as the ritual proceeds. The gateway opens: a shimmering portal in the floor reveals a vast red-lit landscape far below — the inner surface of the world. The party descends.
Welcome to the Hollow World
The party emerges on the inner surface of Mystara — a world where a small red sun hangs motionless in the center of the sky, there is no night, and every direction curves upward. They encounter living Nithians — bronze-skinned people in Egyptian-style garb who build pyramids and worship the sun. The Nithians have no concept of an "outer world" and are wary of the strange newcomers. The party must earn their trust to learn about the cult's activities here. The Nithians speak of dark priests who arrived recently, preaching entropy and death.
The Cult's Plan Revealed
The cult of Thanatos has established a base in an Azcan (Aztec-inspired) pyramid city, where they have corrupted the local priesthood into supporting their plan: using the five keys in a reverse ritual at the Hollow World's central sun to collapse the barrier between surfaces. This would crush the Hollow World and tear the outer surface apart, achieving Thanatos's goal of universal entropy. The party must unite the Nithians, Azcans, and other Hollow World peoples against the cult — a difficult task given the Immortals' spell of isolation that keeps these cultures suspicious of outsiders.
The Battle at the World's Heart
The climactic finale. The cult begins the reverse ritual at a massive pyramid aligned with the Hollow World's central sun. Reality warps: gravity shifts, the inner surface cracks, and the red sun pulses ominously. The party storms the pyramid with Nithian and Azcan allies, fighting through cult fanatics, undead guardians, and a death knight who serves as Thanatos's champion. At the summit, the cult leader channels entropy itself — the party must interrupt the ritual by removing the five keys from their pedestals while fighting the death knight. Success stabilizes both worlds; the gateway between them can be sealed or left open, at the party's choice.
NPCs
Scholar Claransa
Human · Historian and Sage
A Thyatian-born scholar living in Threshold who has spent her life studying historical anomalies — gaps in the record where civilizations should exist but don't. Her discovery that the Nithian hieroglyphs are genuine vindication of decades of dismissed theories terrifies her as much as it excites her. She is brilliant, obsessive, and physically frail, preferring to support the party with knowledge rather than combat.
Baron Sherlane Halaran
Human · Baron of Threshold
The just and capable ruler of Threshold, a Thyatian nobleman who genuinely cares about both the colonist and native populations. He is a former adventurer himself and recognizes the significance of the underground discovery immediately. He funds the party's expedition and provides letters of introduction to contacts across the Known World, but he cannot leave his barony undefended.
Jarl Sigurd Helmgaard
Human · Ostland Chieftain
A towering, red-bearded Viking warrior who rules his clan through strength and generosity. He is deeply superstitious and distrustful of magic, but he respects courage and physical prowess above all else. He knows the location of the sunken longship but will only share it with those who prove themselves in the traditional trials. He has a booming laugh and a surprisingly gentle nature with children and animals.
Prince Morphail Gorevitch-Woszlany
Human (Vampire) · Glantrian Wizard-Prince
One of the ruling wizard-princes of Glantri who secretly harbors the Nithian key in the Great School's vault. He is actually a vampire who has maintained his position through centuries of political manipulation and powerful illusion magic. He will trade the key for a dangerous favor — retrieving a magical tome from a rival prince's library. His charming demeanor hides a predator's patience and cruelty.
Khalid al-Azrad
Human · Desert Guide
A weathered Ylari guide who knows the deep desert better than anyone alive. He has stumbled across Nithian ruins before and considers them cursed. He is devoutly religious, following the teachings of al-Kalim, and he prays five times daily even in the middle of a sandstorm. Despite his superstitious caution, his survival skills and desert knowledge are indispensable for reaching the buried temple.
High Priestess Aketheti
Human · Nithian High Priestess
The spiritual leader of the Hollow World Nithians, a regal woman in gold and lapis lazuli who serves the sun-god. She is initially hostile to the party — outsiders are an anomaly in her world — but the cult of Thanatos's corruption of the Azcan priesthood alarms her enough to consider an alliance. She commands powerful divine magic and her word carries absolute authority among her people.
Locations
Threshold
A frontier logging town in the Grand Duchy of Karameikos, built where the Windrush River meets the Foamfire Valley. Wooden palisades surround log buildings and muddy streets. Traladaran natives and Thyatian colonists live side by side in uneasy coexistence. The town sits atop ancient ruins that predate both cultures — ruins that have just been accidentally unsealed by a mine collapse.
The Northlands — Ostland
A rugged land of fjords, pine forests, and volcanic islands inspired by Viking Scandinavia. Longships ply the cold waters, and warrior-clans compete for glory and territory. The sea is rich with fish but also with monsters — krakens, sea serpents, and draugr ships crewed by the restless dead.
The Principalities of Glantri
A confederation of wizard-ruled principalities nestled in a mountainous region. Magic is law here — noble houses are determined by arcane power, and clerical magic is outlawed. The capital city features canals instead of streets, gondolas instead of carriages, and towers that float unsupported above the skyline. Magical duels between students are a spectator sport.
The Emirate of Ylaruam
A vast desert kingdom inspired by the Arabian Nights, ruled by the philosophical teachings of the Eternal Truth. Oasis cities gleam with white stone and blue tile, while the deep desert hides ancient ruins, nomadic tribes, and creatures adapted to the brutal heat. Sandstorms can bury a caravan in minutes, and the desert is said to drive those who wander it too long to madness.
Thyatis City
The capital of the Thyatian Empire, a sprawling metropolis modeled on ancient Rome. Grand colonnaded avenues, a massive gladiatorial arena, marble senate buildings, and aqueducts define the skyline. The city buzzes with political intrigue, military parades, and the constant jockeying for power among the senatorial families. Wealth and corruption walk hand in hand.
The Hollow World
The inner surface of Mystara — a vast concave landscape lit by a small, stationary red sun at the planet's core. There is no night, no seasons, and the horizon curves upward in every direction. Preserved civilizations — Nithians, Azcans, Oltecs, and others — live here in cultures frozen in time by the Immortals' magic. The landscape is tropical and fertile, with dinosaurs roaming the jungles and ancient cities rising from the canopy.
Quests
Assemble the Five Nithian Keys
Five golden scarab artifacts, scattered across the Known World by forces unknown, must be reunited to open the gateway to the Hollow World. Each key is hidden in a different culture and protected by different dangers. The cult of Thanatos races to collect them first, and they have a significant head start.
Uncover the Truth of Nithia
A civilization powerful enough that the Immortals themselves erased it from history — what did the Nithians do that warranted such total obliteration? Fragments of the truth are scattered in ruins, libraries, and the memories of Immortal servants. Understanding what Nithia was may be the key to understanding what the cult of Thanatos truly intends.
Stop the Cult of Thanatos
The cult of Thanatos, Immortal of Death and Entropy, seeks to use the Nithian keys to collapse the barrier between the outer and inner surfaces of Mystara, destroying both. Their agents are embedded in every culture of the Known World, and their leader — a death knight of terrible power — serves Thanatos directly. They must be stopped before they reach the Hollow World's heart.
Unite the Hollow World Nations
The preserved civilizations of the Hollow World — Nithians, Azcans, Oltecs, and others — are kept isolated from each other by the Immortals' magic. They are suspicious of outsiders and hostile to each other. To mount an effective defense against the cult of Thanatos, the party must somehow convince these ancient peoples to set aside centuries of engineered distrust and fight together.
Encounters
Basilisk in the Mines
A basilisk has wandered into the Nithian chambers through the mine collapse, drawn by the warmth of the magical scarab. It lurks in the darkened tunnels, its petrifying gaze a deadly surprise for anyone carrying a torch. The cramped mine tunnels make it impossible to avoid its line of sight without creative tactics.
Draugr of the Sunken Longship
The undead crew of King Cnute's longship rise from their stations as the party approaches the prow. Waterlogged but relentless, the draugr fight with rusted axes and a cold fury, their glowing blue eyes piercing the murky depths. The draugr captain is a formidable wight who commands the others with barked orders in ancient Norse.
Medusa of the Buried Temple
The inner sanctum of the Ylari desert temple is a gallery of horrifyingly lifelike stone statues — the medusa's collection of petrified adventurers. She has adorned them with jewelry and posed them in dramatic tableaux. She fights with her petrifying gaze and a longbow coated in poison, using her stone garden as cover.
Death Knight of Thanatos
Atop the Azcan pyramid during the climactic ritual, the cult's champion stands revealed: a death knight sworn to Thanatos, Immortal of Entropy. Clad in corroded black armor and wreathed in necrotic flame, it wields a greatsword that drains life force with every strike. It commands undead reinforcements and can unleash a devastating hellfire orb that incinerates everything in its blast radius.
Factions
The Grand Duchy of Karameikos
Lawful Good
A frontier nation where Thyatian colonists rule over native Traladaran populations. Duke Stefan Karameikos is a just ruler trying to forge a unified nation from two cultures that distrust each other. The duchy's military is small but professional, and the duke supports the party's investigation because the Nithian ruins threaten the stability of his realm.
The Cult of Thanatos
Neutral Evil
A secretive death cult devoted to Thanatos, the Immortal of entropy, undeath, and destruction. They believe all existence is a mistake that must be corrected through total annihilation. Their agents infiltrate governments, temples, and academies across the Known World, working toward the day when the barrier between worlds collapses and everything is unmade. They are patient, fanatical, and willing to sacrifice anything — including themselves.
The Hollow World Nithians
Lawful Neutral
The preserved Nithian civilization within the Hollow World — a bronze-age culture of pyramid-builders, sun-worshippers, and powerful divine spellcasters. They have no knowledge of the outer world and believe their red-lit realm is all that exists. Their high priestess Aketheti leads with wisdom and absolute authority, and their warriors fight with khopesh swords and divine magic.
Recommended Characters
Pre-built characters designed for this campaign setting. Create them separately from the character templates page.
Gaius Aurelian
Human · Fighter · Level 3
A disciplined Thyatian legionnaire stationed at a frontier outpost in the Known World. Trained in the rigid military traditions of the Thyatian Empire, Gaius seeks glory and promotion through conquest of the wild borderlands.
Milo Thorngage
Halfling · Rogue · Level 3
A quick-witted halfling scout from the Five Shires, skilled in navigating the wilderness paths and hidden trails of Mystara. Milo's curiosity about the wider Known World has drawn him far from the safety of his homeland.
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