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 focus the hunger of Gorathax the Ship-Render toward his rivals. Kaspar does not command the leviathan; he merely feeds it, and each feeding weakens the ancient ward-stones imprisoning 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 — Governor Ernst von Grauberg’s colonial commission, rival pirate captain Brunhilde “the Iron Widow” Kesselring, and independent action — each shaping what ship, what crew, and what enemies the party carries into the sea. Kaspar’s own first mate, Klaus Mordstein, is quietly engineering a mutiny that the party may ignite or extinguish. The adventure is divided into four chapters.
Chapter I: Port Mordham
Port Mordham hums with dread: lanterns sputter in the dusk, sailors make warding gestures against the sea, and no one answers the merchant captains offering absurd wages to sail the Widow’s Reach. The Rotting Docks serve as the faction crossroads — the governor’s stiff-backed courier and Kesselring’s scarred boatswain both make their pitches here, while dock gossip first surfaces Klaus’s name as the man with the courage to whisper against Kaspar. The Scurvy Rat Tavern is the adventure’s most information-dense location: barkeep Three-Finger Gottfried sells the bearings of Deadman’s Reef for fifty gold or a dangerous favor, and traumatized survivor Captain Dietrich “The Gull” Hafner — the only man to watch Gorathax surface and live — describes tentacles as thick as masts and copper-burning eyes, and reveals the five ward-stone pillars encircling the reef from Kaspar’s own charts. Four optional locations round out the chapter: the Harbor Watch Post (Harbormaster Mathilde Rottermund, ship movement logs, navigation advantage), Blackpowder Alley (paranoid armsmith Gustav Kreutzmann and his prototype anti-leviathan Harpoon Gun), Gibbet Square (raving Father Aldrich preaching over barnacle-encrusted naval corpses, ward-stone lore, party-wide inspiration), and The Press Gang (Kesselring’s forced recruitment encounter, triggering only for parties who haven’t allied with her). Fail-forward mechanics ensure critical intelligence about the Reef and the ward-stones reaches the party regardless of which locations they visit.
Chapter II: The Bloody Waters
The Widow’s Reach punishes caution and ignorance equally. Six sea encounters unfold across the 12-hour base crossing: the Wreck of the Sea Serpent (barnacle-infected wreckage confirming Gorathax’s biological corruption), the False Calm (ominous stillness, massive shadow below, crew morale crisis), the Red Storm (a supernatural tempest tied to a ritual surge, requiring coordinated ship-action skill checks to survive), the Drifting Hulk (Kesselring’s shattered brig with infected survivors who may reinforce the crew or spread panic), the Tentacle Skirmish (a projection of Gorathax testing the hull, confirming it is incompletely bound), and The Whispering Sea (the ocean calling sailors by name, one crewman walking toward the rail in his sleep). At the chapter’s end the party faces a choice: strike directly for Deadman’s Reef and gamble on Gorathax being weakened, make for Grotburg Isle to exploit the mutiny and arm themselves with intelligence, or push deeper into the Reach for more. The Ritual Clock — with between 27 and 45 hours typically remaining — shapes which path is viable.
Chapter III: Grotburg Isle
Kaspar’s volcanic fortress island is a cage, not a haven. Forty pirates drink too fast and jump at shadows; twenty of Klaus’s loyalists wait for a spark. Kaspar himself is already at the Reef. Four approaches unlock the island’s secrets: direct assault (fast, bloody, reveals nothing), stealth infiltration (sea cave, harbor chains, DC 16 thieves’ tools), social manipulation (DC 13 Deception with stolen garb or passwords), or a coordinated split operation. Four mapped locations reward thorough investigation: The Bleeding Hall (drinking contests, card games, fistfights to earn pirate respect; DC 14 Charisma to start a brawl that clears guards), The Shrine of the Depths (the open Tome of Tidal Whispers, a stagnant pool that floods contactees with Gorathax’s hunger on a failed DC 15 Wisdom save, and pictographs proving Kaspar worships rather than commands), Kaspar’s Quarters (paranoid logs charting his descent, a loaded Conch of Calling on a pedestal that alerts Kaspar if blown, and Klaus’s hidden translation notes confirming the ritual’s true mechanics), and The Armory (Klaus’s secret council chamber, alchemical firebombs, charts of Deadman’s Reef, a locked 300 gp cache). A three-way standoff between the party, Klaus, and Kesselring may erupt in the Bleeding Hall or the Armory — resolved by negotiation, intimidation, deception, or full combat with powder-keg environmental hazards. By departure, the party knows the ritual’s mechanics, the ward-stone 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 four miles of escalating dread — dead fish, muffled sound, phosphorescent ghost-ships, and gravity that feels wrong. Environmental hazards include jagged reefs (DC 13 Survival to navigate), sudden whirlpools (DC 14 group Strength to break free), blood fog (DC 12 Constitution save or attack disadvantage), and dead coral knives for the overboard. Five weathered ward-stone pillars ring the reef; each requires one minute of focused work and a DC 16 Arcana check (or 5th-level dispel magic) to activate, each attempt provoking a tentacle strike at +10. Progressive activation strips Gorathax of legendary resistances, legendary actions, lair actions, and up to 120 HP — but costs precious clock time. Kaspar fights from a coral ritual platform using the Tome and the Conch, defended by summoned animated tentacles, with a Blood Bond ability halving all damage while Gorathax lives; at 30 HP he begins a soul-binding ritual that, if completed, merges him with the creature permanently. Gorathax surfaces as a living coastline of muscle and stone-white suckers — tentacles at +10 reaching 20 feet, a bite that can swallow Large-or-smaller targets for 3d6 acid per round, lair surges on initiative 20 (tidal waves, quipper swarms, whirlpools), and a phase change below half HP granting attack advantage at the cost of defense. Four legitimate resolution paths exist: Reinforce the Prison (all five stones plus a one-hour DC 18 group Arcana ritual under assault), Banishment (7th-level spell using the Tome as focus, DC 20 Concentration saves each minute for ten minutes), Negotiated Accord (three-step Charisma negotiation at DC 16/18/20 in Abyssal or Primordial, with a suitable offering), or Destructive Gambit (Klaus’s explosives placed at marked reef channels while the party distracts the leviathan). Six epilogue outcomes — combat victory, renewed prison, banishment, negotiated peace, pyrrhic collapse, or catastrophic failure — each carry full faction consequences: Governor von Grauberg grants letters of marque and 500 gp per character (or files bounties if opposed); Kesselring claims Grotburg or fractures into a power vacuum; Klaus either reforms the archipelago’s piracy or vanishes to nurse a grudge. Base treasure totals 2,500 gp with conditional bonuses up to an additional 1,200 gp from bounties, official payments, and faction profit shares.
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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