And Here I have a monster for you to throw at your players. Players in this game should stay out.
It is not from older editions of D&D but from classic greek myth. I'm not so sure on the beasts cr though. For on one hand it is a very very nasty in melee. On the other hand it has no defense against spells. When your party gets highlevel you can throw this at their ship.
Sea Serpent
A classic monster the sea serpent is a ravenous beast shaped like a huge aquatic snake that feeds on the sailors of ships.
Sea Serpent
Gargantuan Animal (Aquatic)
Hitpoints: 24d8+168 (276)
Initiative: +4
Speed: Swim 40 ft (8 squares)
AC: 32 (+26 natural, -4 size) Flatfooted 32, Touch 6
Base Attack/Grapple: +18/+43
Attack: Bite +28 (4d6+19/ 19-20 x2) melee
Full Attack: Bite +28 (4d6+19/ 19-20 x2) melee
Face/Reach: 20 ft/15 ft
Special Attacks: Lunge, Frightful Presence
Special Qualities: Darkvison 60ft, Lowlight Vision
Saves: Fort +22, Reflex +15, Will +10
Abilities: Str 37, Dex 10, Con 24, Int 1, Wis 10, Cha 10,
Skills: Spot +14, Listen +14
Feats: Power Attack. Cleave, Great Cleave, Weapon Focus Bite, Improved Critical Bite, Improve Initiative, Iron Will, Lighting Reflexes, Great Fortitude
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Climate: Any aquatic
Organization: Solitary or Pair
Challenge Rating: 14
Treasure: None
Alignment: Always neutral
Advancement: 25-32(Gargantuan), 33-72(Colossal)
Sea Serpents are ravenous beasts the solely exist to hunt for food, and prey on the sailors out of ships.
A sea serpent appears as a very large blue scaled snake the lives in the water. It often is seen with portions of the middle of its body underwater and the tail is almost always under the surface. Most ocean animals avoid the sea serpent or they get eaten.
Combat: A sea serpent starts combat by lunging up to the ships deck and attacking the nearest person. It will continue to repeat attacking this same person until that person goes out of reach or it is driven to low hit points. Once a target dies it will start on the next.
Lunge (ex): On the first round of combat a Sea Serpent may lunge out of the water and attack any one on the deck of a ship for a extra damaging bite attack. This bite attack does 4d8+26 do to the quick motion.
Frightful Presence(ex): Upon witnessing a lunge attack all creatures within 60 ft much make a will save dc:22 or become frightened as the state described on page 301 of the Dmg. On a successful save that creature is immune to the Frightful presence of that particular Sea Serpent for 24 hours.