Captain Kaspar Fleischacker has made a pact with Gorathax the Ship-Render, an ancient leviathan imprisoned beneath Deadman’s Reef — feeding it ships and crews to terrorize the Grimhook Archipelago — and the 72-hour ritual clock has already started ticking.
Welcome to “Under the Black Flag” 5E adventure! Within these pages, you’ll discover everything necessary to embark on this thrilling quest, complete with detailed descriptions of locations, encounters, NPCs and stat blocks, maps, and handouts.
This adventure also features a versatile and scalable system, allowing Game Masters to easily adjust the difficulty and complexity of encounters to suit their players’ preferences and skill levels. Whether you have a party of advanced players or beginners, you can tailor the challenges to ensure an exciting and engaging experience for everyone involved. The scalable system guides modifying creature statistics, making it simple to customize the adventure to fit your party’s level.
In “Under the Black Flag,” a party of 4-6 characters (levels 5-8) arrives in the pirate port of Port Mordham to find the Grimhook Archipelago’s trade routes strangled by Kaspar Fleischacker — a captain who bartered his soul, and other people’s crews, to turn the hunger of Gorathax the Ship-Render against his rivals. Kaspar does not command the leviathan; he merely feeds it, and each feeding weakens the ancient ward-stones that imprison it beneath Deadman’s Reef. The moment the party sets foot in Mordham, a 72-hour Ritual Clock begins counting down to the final feeding that will break Gorathax free entirely. Three factions compete for the party’s allegiance — a colonial governor’s commission, a rival pirate captain known as the Iron Widow, and independent action — each shaping what ship, what crew, and what enemies the party carries to sea. And Kaspar’s own first mate, Klaus Mordstein, is quietly engineering a mutiny the party may ignite or extinguish. The adventure is divided into four chapters.
Chapter I: Port Mordham
Port Mordham hums with dread — guttering lanterns, sailors making warding gestures against the sea, merchant captains offering absurd wages for crews who will not sail. The Rotting Docks serve as the faction crossroads, where the governor’s and the Iron Widow’s agents both make their pitches and dock gossip first surfaces Klaus’s name. The Scurvy Rat Tavern is the chapter’s richest vein of information, home to a barkeep who sells dangerous bearings and the only survivor to watch Gorathax surface and live. A handful of optional locations — a harbor watch post, a paranoid armsmith’s workshop, a square where a mad priest preaches over barnacled corpses, and a press-gang encounter for parties who haven’t picked a side — round out the port. Fail-forward design ensures the critical intelligence about the Reef and the ward-stones reaches the party no matter where they wander.
Chapter II: The Bloody Waters
The Widow’s Reach punishes caution and ignorance equally. A series of sea encounters unfolds across the crossing — a barnacle-infected wreck, an ominous false calm, a supernatural storm tied to the ritual, a drifting hulk of infected survivors, a probing strike from Gorathax itself, and a stretch of ocean that calls sailors by name in their sleep. At the chapter’s close the party must choose a course: strike directly for Deadman’s Reef and gamble on the leviathan being weakened, make for Kaspar’s island to exploit the brewing mutiny, or push deeper into the Reach. The Ritual Clock decides which gambles are still affordable.
Chapter III: Grotburg Isle
Kaspar’s volcanic fortress island is a cage, not a haven — restless pirates jumping at shadows, a body of mutineers waiting for a spark, and Kaspar himself already gone ahead to the Reef. The party can take the island by direct assault, stealth, social manipulation, or a coordinated split operation, and several mapped locations reward thorough investigation: a hall where pirate respect is earned over drinks and fistfights, a shrine that proves Kaspar worships the leviathan rather than commands it, his private quarters and their evidence of his unraveling mind, and a hidden armory that doubles as the mutineers’ council chamber. A three-way standoff between the party, the mutineers, and the Iron Widow may erupt before the chapter ends — settled by negotiation, intimidation, deception, or open combat amid powder-keg hazards. By departure, the party knows the ritual’s mechanics, the ward-stones’ locations, and exactly how much time remains.
Chapter IV: The Blood Tide
The ocean thickens to blood-red as the party approaches Deadman’s Reef through miles of escalating dread — dead fish, muffled sound, phosphorescent ghost-ships, gravity that feels wrong — past reefs, whirlpools, and blood fog. Five weathered ward-stone pillars ring the reef, and each one the party can restore strips Gorathax of some of its power before the final confrontation — though every attempt invites the leviathan’s wrath, and every minute spent burns the clock. Kaspar fights from a coral ritual platform, shielded by a Blood Bond that protects him so long as Gorathax lives, and racing to complete a soul-binding ritual that would merge him with the creature forever. Gorathax surfaces as a living coastline of muscle and stone-white suckers, capable of swallowing whole sailors and reshaping the battle around it. The climax offers four legitimate resolutions — reinforce the ancient prison, banish the leviathan back to the deep, negotiate an accord with a being older than the archipelago, or stake everything on a destructive gambit — and the adventure closes with a range of epilogue outcomes, each carrying full faction consequences for the governor, the Iron Widow, and the mutineers, along with rewards that scale to the alliances kept and broken along the way.
Release Date: 2025 Format: Digital (PDF)
Items Included
- Under the Black Flag 5E (PDF)
- A B&W Printer-Friendly Version (PDF)
- Maps
- Illustrations (Generated with Midjourney)
- Monster Stat Blocks
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