Karkinos at the Keel
One-Shot · 6 Players · Level 8 · ~3–3.5 Hours · Single Trireme · Sacred Crab
📜 Session Summary
| Style Single-ship sea threat — Beasts → Fey parley → Sacred crab boss |
| Win Condition Drive off or defeat Karkinos (0 HP, or ≤30 HP after Coral Feast-Horn) |
| Players 6 × Level 8 · Duration 3–3.5 hrs · Board: party trireme only |
| Minis Shark (≤3), Octopus (1), Sea spirit (≤5), Sea/Water Nymph (1), Dolphin (1), Cancer the Crab (1) |
Description
A Nereid herald boards through Misthaven mist with the Coral Feast-Horn. Blood-chum in the water has drawn a sacred crab mad. All three encounters play on and around one hull — no enemy ship.
Trireme Map (Single Ship)
Main deck 40×50 ft · raised bow +15 ft · raised stern +25 ft · 90 ft total length (tapered ends) · center mast/sail · 15 ft ram at bow. No second ship — water, sky, and creatures only. Place shark/octopus minis in water adjacent to hull; Karkinos climbs the ram or stern in Enc. 3.
Major Beats
- Mist-trireme: herald + Coral Feast-Horn.
- Enc. 1: Sharks + octopus thrash the oars.
- Enc. 2: Thalassa’s shoal — parley / recruit (+ optional Delphinus).
- Enc. 3: Karkinos climbs aboard; Horn payoff.
Creature / Enemy List
| Creature | Easy | Med | Hard | Notes |
|---|---|---|---|---|
| Hunting Shark | 2 | 3 | 3 | Enc. 1 |
| Giant Octopus | 0 | 1 | 1 | Enc. 1 |
| Thalassa | — | 1 | 1 | Enc. 2 recruit |
| Sea Spirit | 0–2 | 0–3 | 0–5 | Optional |
| Karkinos | 1 adj | 1 | 1 | Boss |
Encounter 1 — Blood in the Oar-Wash
Overview
3× Hunting Sharks and 1× Giant Octopus boil up from chum-stained water. Win: clear the beasts; keep oars usable (≥2 oar banks intact).
🗺 Terrain
Single trireme + 15 ft water ring. Octopus prefers stern/oar ports; sharks circle the ram.
Mechanics
- Oar banks: 4 zones (AC 12 / HP 12). If ≥3 destroyed → Short Rest only before Enc. 2; else Long Rest.
- Non-combat: Coral Feast-Horn + Animal Handling/Religion group challenge — 3 successes before 3 failures (DC 13) drives sharks off; octopus still must be cut free or contested Ath DC 14.
- Overboard: Ath DC 12 to climb ram/oar-ports; sharks get OA.
Creature Cards
Hunting Shark
Large Beast, Unaligned · CR 2 · Mini ×3
AC 12 | HP 45 (6d10+12) | Speed 0 ft., swim 40 ft.
Senses: blindsight 30 ft. · Blood Frenzy: adv vs. creatures missing HP
Hull Circler. Can make opportunity attacks against creatures that leave the water within 5 ft. of the hull or ram.
Bite. +6 to hit. Hit: 13 (2d8+4) piercing.
Giant Octopus
Large Beast, Unaligned · CR 1 · Mini ×1
AC 11 | HP 52 (8d10+8) | Speed 10 ft., swim 60 ft.
Skills: Stealth +5 · Hold Breath 1 hour on land
Oar Grapple. While grappling the hull/oars, the ship’s row Speed is 0 until freed (Athletics DC 14 as action, or deal 15 damage to tentacles treated as object AC 10).
Tentacles. +5 to hit, reach 15 ft. Hit: 10 (2d6+3) bludgeoning + grapple (escape DC 16).
Ink Cloud (Recharge 4–6). 20-ft radius underwater heavily obscured 1 minute.
Coral Feast-Horn
Wondrous Item (maguffin), Unique
Appearance: small coral trumpet, warm and faintly pulsing
Handout (22 words). "When the sacred crab climbs for blood that is not the sea’s, sound this horn — hunger hears the true feast."
Enc. 1–2. Action: blow once (1/short rest) — sharks or octopus within 30 ft. make a DC 13 Wisdom save or end their next turn moving away from the hull.
Enc. 3. Action adjacent to Karkinos or at the bow rail: sound the true feast (once). Karkinos must succeed DC 15 Wisdom or be incapacitated until end of its next turn and lose Legendary Actions that round; soft underbelly exposed (attacks vs. it have advantage until it moves).
Encounter 2 — The Nymph’s Shoal
Overview
Thalassa rises with 3× Sea Spirits. She demands the false feast-horn the poachers used — the party has the true one. Calm her, return a pearl-net tangled on the ram, or fight briefly.
🗺 Terrain
Still water around the hull; light fog. No second ship.
Mechanics
- Non-combat path: Show Coral Feast-Horn + Persuasion/Religion DC 14 (Insight DC 12 for advantage) → she believes you. Return pearl-net (Investigation DC 12 on ram) drops DC to 10.
- Combat path: Thalassa + spirits for 2 rounds or until she hits ½ HP, then flees.
- RECRUITMENT: Peaceful resolution → Thalassa aids Enc. 3; Delphinus arrives as omen.
Creature Cards
Thalassa (Sea Nymph)
Medium Fey, Neutral · CR 3 · Mini: Sea/Water Nymph ×1
AC 14 | HP 58 (9d8+18) | Speed 30 ft., swim 40 ft.
Spell DC 14 · fog cloud, control water (1/day), healing word
RECRUITMENT. Spare her shoal / return a stolen pearl-net (skill path) → she joins Enc. 3: once casts healing word and can use Help to expose Karkinos’s underbelly.
Anger Path. If the party attacks first, she fights 2 rounds then flees; no ally in Enc. 3.
Multiattack. Two tidal darts.
Tidal Dart. +5 to hit, range 60 ft. Hit: 8 (1d8+3) bludgeoning + shove 5 ft.
Charming Glance (1/day). One creature DC 14 Wisdom or charmed 1 minute.
Sea Spirit (Lesser Nereid)
Medium Fey, Chaotic Neutral · CR 1/2 · Mini ×5
AC 12 | HP 22 (5d8) | Speed 30 ft., swim 30 ft.
Optional fog clutter / messengers
Mist Form. Bonus action Hide while lightly obscured by spray.
RECRUITMENT. If not attacked: Persuasion DC 12 → one spirit warns when Karkinos will surface (party cannot be surprised in Enc. 3).
Salt Slap. +4 to hit. Hit: 5 (1d6+2) bludgeoning.
Delphinus (Dolphin Ally)
Medium Beast, Neutral Good · CR 1/2 · Mini ×1
AC 12 | HP 27 | Speed 0 ft., swim 60 ft.
Poseidon’s omen — appears if Thalassa is recruited or horn blown in Enc. 2
Rescue Breach. Reaction: when a PC falls overboard within 20 ft., Delphinus moves and the PC may board as part of that reaction (Ath DC 10).
Ram. +4 to hit. Hit: 7 (1d8+2) bludgeoning.
Encounter 3 — Karkinos Climbs (Boss)
Overview
Karkinos heaves onto the bow via the 15 ft ram, then scuttles toward the mast. Optional: 1 shark remains in water (if Enc. 1 leftover).
🗺 Terrain
Full 90 ft deck. Mast = climb perch. Ram = difficult terrain entry lane.
Mechanics
- Win: 0 HP or ≤30 HP after a successful Coral Feast-Horn blow this fight (it plunges away).
- Horn payoff: incapacitate 1 round + soft underbelly (see item card).
- Ally: Thalassa Help / healing word; Delphinus rescues overboard PCs.
Creature Cards
Karkinos, Sacred Crab of Poseidon
Huge Monstrosity, Neutral · CR 9 · BOSS · Mini: Cancer the Crab
AC 18 (natural) | HP 168 (16d12+64) | Speed 30 ft., swim 40 ft., climb 20 ft.
Saves: Str +10, Con +8 · Resist: bludgeoning nonmagical · LR 2/day
Climb Hull. Treat the trireme as climbable terrain; can end turns on main/bow/stern decks.
Siege Monster. Double damage to objects (rails, mast step, ram housing).
Soft Underbelly. While prone, restrained, or after Coral Feast-Horn success: attacks have advantage; its AC is 15 until start of its next turn.
Persistence. Sacred beast — driving it off at ≤30 HP with the Horn counts as victory; killing it is allowed but Nereids mourn.
Multiattack. Two claws. If both hit the same target, it is grappled (escape DC 16).
Claw. +10 to hit, reach 10 ft. Hit: 16 (2d8+6) bludgeoning.
Pincer Crush (Recharge 5–6). One grappled creature DC 16 Strength or 22 (4d8+4) bludgeoning and restrained while grappled.
Deck Slam. Each creature in a 10-ft radius on deck Dex DC 16 or 14 (4d6) bludgeoning and prone.
Detect (1). Wisdom (Perception) check.
Claw (1). One claw attack.
Scuttle (2). Move up to half speed without provoking; may climb.
Coral Feast-Horn
Wondrous Item (maguffin), Unique
Appearance: small coral trumpet, warm and faintly pulsing
Handout (22 words). "When the sacred crab climbs for blood that is not the sea’s, sound this horn — hunger hears the true feast."
Enc. 1–2. Action: blow once (1/short rest) — sharks or octopus within 30 ft. make a DC 13 Wisdom save or end their next turn moving away from the hull.
Enc. 3. Action adjacent to Karkinos or at the bow rail: sound the true feast (once). Karkinos must succeed DC 15 Wisdom or be incapacitated until end of its next turn and lose Legendary Actions that round; soft underbelly exposed (attacks vs. it have advantage until it moves).
Thalassa (Sea Nymph)
Medium Fey, Neutral · CR 3 · Mini: Sea/Water Nymph ×1
AC 14 | HP 58 (9d8+18) | Speed 30 ft., swim 40 ft.
Spell DC 14 · fog cloud, control water (1/day), healing word
RECRUITMENT. Spare her shoal / return a stolen pearl-net (skill path) → she joins Enc. 3: once casts healing word and can use Help to expose Karkinos’s underbelly.
Anger Path. If the party attacks first, she fights 2 rounds then flees; no ally in Enc. 3.
Multiattack. Two tidal darts.
Tidal Dart. +5 to hit, range 60 ft. Hit: 8 (1d8+3) bludgeoning + shove 5 ft.
Charming Glance (1/day). One creature DC 14 Wisdom or charmed 1 minute.
⚖️ Difficulty Scaling
As Written ★ — 6 Players, Average Level 8
- Baseline for this guide. Numbers, DCs, and win tracks assume six PCs at average level 8.
Library Standard — 6 Players, Average Level 8
- Identical to As Written — this guide’s baseline is already 6 PCs at average level 8.
Easier
- APL 4–6: reduce minion counts ~30%; DCs −2; boss loses one recharge or legendary option.
Harder
- APL 10–12: boss HP +25–35%; +1 legendary action; add a lieutenant or extra wave.
📋 Appendix — Cards
Nereid Herald (Quest Giver)
Medium Fey, Neutral Good · Quest Giver
Public face: salt-crowned messenger stepping from the mist onto the deck
Motivation. A sacred crab of Poseidon — Karkinos — has been goaded from the deep by poachers’ blood-chum. Calm or defeat it before it cracks the next hull.
Secret. The poachers used a false “feast-horn” of Poseidon; the true Coral Feast-Horn reverses the frenzy.
Coral Feast-Horn
Wondrous Item (maguffin), Unique
Appearance: small coral trumpet, warm and faintly pulsing
Handout (22 words). "When the sacred crab climbs for blood that is not the sea’s, sound this horn — hunger hears the true feast."
Enc. 1–2. Action: blow once (1/short rest) — sharks or octopus within 30 ft. make a DC 13 Wisdom save or end their next turn moving away from the hull.
Enc. 3. Action adjacent to Karkinos or at the bow rail: sound the true feast (once). Karkinos must succeed DC 15 Wisdom or be incapacitated until end of its next turn and lose Legendary Actions that round; soft underbelly exposed (attacks vs. it have advantage until it moves).
Hunting Shark
Large Beast, Unaligned · CR 2 · Mini ×3
AC 12 | HP 45 (6d10+12) | Speed 0 ft., swim 40 ft.
Senses: blindsight 30 ft. · Blood Frenzy: adv vs. creatures missing HP
Hull Circler. Can make opportunity attacks against creatures that leave the water within 5 ft. of the hull or ram.
Bite. +6 to hit. Hit: 13 (2d8+4) piercing.
Giant Octopus
Large Beast, Unaligned · CR 1 · Mini ×1
AC 11 | HP 52 (8d10+8) | Speed 10 ft., swim 60 ft.
Skills: Stealth +5 · Hold Breath 1 hour on land
Oar Grapple. While grappling the hull/oars, the ship’s row Speed is 0 until freed (Athletics DC 14 as action, or deal 15 damage to tentacles treated as object AC 10).
Tentacles. +5 to hit, reach 15 ft. Hit: 10 (2d6+3) bludgeoning + grapple (escape DC 16).
Ink Cloud (Recharge 4–6). 20-ft radius underwater heavily obscured 1 minute.
Sea Spirit (Lesser Nereid)
Medium Fey, Chaotic Neutral · CR 1/2 · Mini ×5
AC 12 | HP 22 (5d8) | Speed 30 ft., swim 30 ft.
Optional fog clutter / messengers
Mist Form. Bonus action Hide while lightly obscured by spray.
RECRUITMENT. If not attacked: Persuasion DC 12 → one spirit warns when Karkinos will surface (party cannot be surprised in Enc. 3).
Salt Slap. +4 to hit. Hit: 5 (1d6+2) bludgeoning.
Thalassa (Sea Nymph)
Medium Fey, Neutral · CR 3 · Mini: Sea/Water Nymph ×1
AC 14 | HP 58 (9d8+18) | Speed 30 ft., swim 40 ft.
Spell DC 14 · fog cloud, control water (1/day), healing word
RECRUITMENT. Spare her shoal / return a stolen pearl-net (skill path) → she joins Enc. 3: once casts healing word and can use Help to expose Karkinos’s underbelly.
Anger Path. If the party attacks first, she fights 2 rounds then flees; no ally in Enc. 3.
Multiattack. Two tidal darts.
Tidal Dart. +5 to hit, range 60 ft. Hit: 8 (1d8+3) bludgeoning + shove 5 ft.
Charming Glance (1/day). One creature DC 14 Wisdom or charmed 1 minute.
Delphinus (Dolphin Ally)
Medium Beast, Neutral Good · CR 1/2 · Mini ×1
AC 12 | HP 27 | Speed 0 ft., swim 60 ft.
Poseidon’s omen — appears if Thalassa is recruited or horn blown in Enc. 2
Rescue Breach. Reaction: when a PC falls overboard within 20 ft., Delphinus moves and the PC may board as part of that reaction (Ath DC 10).
Ram. +4 to hit. Hit: 7 (1d8+2) bludgeoning.
Karkinos, Sacred Crab of Poseidon
Huge Monstrosity, Neutral · CR 9 · BOSS · Mini: Cancer the Crab
AC 18 (natural) | HP 168 (16d12+64) | Speed 30 ft., swim 40 ft., climb 20 ft.
Saves: Str +10, Con +8 · Resist: bludgeoning nonmagical · LR 2/day
Climb Hull. Treat the trireme as climbable terrain; can end turns on main/bow/stern decks.
Siege Monster. Double damage to objects (rails, mast step, ram housing).
Soft Underbelly. While prone, restrained, or after Coral Feast-Horn success: attacks have advantage; its AC is 15 until start of its next turn.
Persistence. Sacred beast — driving it off at ≤30 HP with the Horn counts as victory; killing it is allowed but Nereids mourn.
Multiattack. Two claws. If both hit the same target, it is grappled (escape DC 16).
Claw. +10 to hit, reach 10 ft. Hit: 16 (2d8+6) bludgeoning.
Pincer Crush (Recharge 5–6). One grappled creature DC 16 Strength or 22 (4d8+4) bludgeoning and restrained while grappled.
Deck Slam. Each creature in a 10-ft radius on deck Dex DC 16 or 14 (4d6) bludgeoning and prone.
Detect (1). Wisdom (Perception) check.
Claw (1). One claw attack.
Scuttle (2). Move up to half speed without provoking; may climb.