I want to tell you about one of my absolute favorite monsters I put into the Monstrous Menagerie, which is available for two more days as part of the Level Up 5E kickstarter. (Go back it!)

That monster is… the sea serpent.

Kind of a weird favorite monster, right? It’s kind of a fringe D&D monster. As far as I know, it hasn’t appeared in a Monster Manual (unless the first edition giant sea snake counts?). It was mentioned as a homebrew possibility in OD&D, and it’s been in a few adventures and supplements and adventures across the editions, including Fizban’s, but it’s never been a core monster. It’s always puzzled me that the sea serpent, arguably the most well-known oceangoing monster in popular culture, has had so little D&D traction.

This won’t do at all. The sea serpent is an iconic and instantly recognizable threat. Furthermore, ocean encounters are sparse enough in D&D. I’m running a high-level nautical campaign and need all the monster variety I can get beyond dragon turtles and marids. I want every sea captain to be just as wary of sea serpents as they are of storms and sea hags.

Before we start talking about design, we should think about the question: why IS there no core sea serpent in D&D? Is there a design pitfall I’m not seeing?

Well, one problem is that sea serpents aren’t adventurer scale. They threaten ships. In the fantasy sea serpent battles I remember best – Voyage of the Dawn Treader, and even Gary Gygax’s Gord the Rogue series – the serpent is trying to destroy the ship, ignoring the crew for the most part. Any monster that can smash a ship to flinders is just the wrong size to fight PCs, right?

Ehh, maybe. But the 5e kraken and tarrasque beg to differ. In D&D, no monster is too big to fight.

In fact, I see sea monsters’ size and tactics an as an opportunity to make them unique among monsters: they can be useful opponents at any level. While most monsters are threats to characters, sea serpents are primarily threats to ships. Since ship statistics don’t change much between character level 1 and 20, that means that serpents are perennial threats. After all, even first-level characters take ship rides. And in D&D, a shipwreck is the beginning of an an adventure, not its end.

Our usage of sea serpents will change over the course of the PCs’ career:

At low levels, sea serpents are a force of nature, like a storm at sea, that threatens shipwreck on the coast of the Isle of Dread. Low-level characters don’t own ships, and they’re too weak to get the sea serpents’ attenion, so all that’s imperiled is the characters’ mission. Still, even the weakest character can point a ballista or score a critical hit that saves the ship!

At medium levels, sea serpents are still primarily a threat to the PCs’ ship, not to the PCs. It’s a chance for a “safe” battle where the stakes are a setback (shipwreck or ship damage) rather than character death. However, since mid-level characters might own their own ship, these stakes can still feel meaningful. And at this level, victory is possible: while the heroes might not be able to kill the monster singlehandedly, they should have a good chance of driving it off with the help of the crew.

At high levels, the sea serpent is just another monster to be killed, like the dragon turtle or kraken. High-level characters can deal enough damage to get the sea serpent’s attention, so it’s likely to be a battle between the sea serpent and the characters, with the ship an afterthought.

I think this could be a good model for ship-bound combat in general, from the high seas to astral piracy to space and beyond: the monster is tailored to fight the ship at low levels, but its attention can be claimed by the characters at high levels.

Now let’s stat out the beast!

First of all, let’s look at ships in 5e D&D: the strength of a typical ship will determine how much damage a sea serpent needs to do. Ship hit points range from 300 (sailing ship) to 500 hit points (warship). If a sea serpent deals 50 damage to a sailing ship each round, that would mean that the PCs have 6 turns to drive the serpent away in order to save their ship. If they can’t do it by then, it ain’t happening.

Here’s what I imagine a sea serpent doing in a turn: