D&D General Suggest to me a dangerous sea monster from any edition

Storm giant.

Maybe a Typhoon Dragon (Tun Mi Lung) or similar?

Sword of Spirit said:

A dragon turtle shouldn't really be summoned from the depths. A typical sea serpent might be, if I want to create a more powerful version. Giants and such are too easy to negotiate with. A kraken seems too dangerous, and I'm still not sure the "young kraken" idea works for the kraken lore I'm leaning towards. The 2e (whale) leviathan is a good guy that shouldn't behave in this manner.

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The way I'm thinking it makes sense that a typhoon dragon might be summoned from the depths is if the conjured storm disturbs it in its palace at the bottom of the ocean. Though that would make it a side effect of the spell, and I'm not sure if you need its summoning to be intentional (and under the control of the summoner?).

If it is meant to be deliberately summoned, and under the summoner's control, does some sort of water elemental fit the bill? (There are lots of them, at various levels of power, across the editions of the game.)

The Sea Dragon from Tome of Beasts 1 (Kobold Press) is pretty cool if you are in to third party. The Morkoth from MPMM is also cool

A lot depends on if the summoning is intentional and controlled or not. There are a great many sea monsters who would be attracted by a magical storm without being directly summoned.

Or go classical - an archfey/air elemental called Ariel.

Is this something the party are expected to fight or not? If it’s role is purely narrative, you want something far to powerful for the PCs to fight - they survive because it’s not interested in them.

If it’s near the PCs in power, it really only ends two ways - the PCs kill it or it kills them.

Sword of Spirit said:

Desired results:
-The monster is escapable (and will not pursue)
-The monster will do some damage to the ship before being escaped (could be a risk of destruction, or just a bit of damage before they escape)
-The 5th-level characters can survive a round or two of combat
-Does not create a massive wave that damages everyone on the ship, since some NPCs should survive (see below)
-The monster isn't something you can negotiate with, and makes sense being summoned by the spell
-The monster will hang around for a week or two along with the storm it's summoned in, to deter going back that direction
-CR probably mid to upper teens, should be something the party knows is beyond them rather than something they think they can take on

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Maybe a Masher?

Paul Farquhar said:

Is this something the party are expected to fight or not? If it’s role is purely narrative, you want something far to powerful for the PCs to fight - they survive because it’s not interested in them.

If it’s near the PCs in power, it really only ends two ways - the PCs kill it or it kills them.

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Ideally, the party realizes it's too dangerous (especially since they have already demonstrated they are invested in keeping the NPC crew safe) and just tries to minimize casualties while escaping.

What I would love the experience to look like is the monster repeatedly damaging the ship while picking off a few crew and injuring the party until the ship manages to travel far enough that it doesn't want to pursue. At the same time, after I set the parameters of a scenario, I'm not going to DM fiat it: things will happen naturally according to the dice and the choices of the PCs, NPCs, and monsters. Of course that means it may not look (or go) much the way I imagine, but that's okay as long as it's a possibility, and we avoid the extremes of an unintentional character death or an overly easy getaway without really feeling the danger.

I suppose I could have it just be a kraken (so it's clearly way too powerful to survive) but appear far enough away that they can entirely avoid engaging if they hightail it out of there immediately, and just set it up so ship damage will come from the storm. I'd be concerned in that case that they might somehow manage to get engaged with it anyway, and that would be bad.

This adventure is more linear than the typically sandboxy ones I run. I never constrain where the PCs go or what approach they can take, and since there are reasons that it's best if they end up with a damaged ship and afraid to go back into this storm, it's taking me a lot of finesse to make that result basically unavoidable without handwaving, or contriving situations that don't make sense in the world. Worse case scenario is I tell the players, "look guys, it's going to be more fun if you go this way rather than that way, so how about you try to think of an in-character reason to do that?" But it's going to feel more satisfying if the world works as it should, the game rules work acceptably, and they choose freely while having having that all results in the desired outcome. So that why I'm looking for just the right monster.

I think this needs a cinematic approach. I wouldn’t use a standard monster. I would use something kaiju sized. A mile high octopus or seaweed monster is awakened from the depths by the summoned storm. The party can't fight it directly (it has a bazzillion hit points) but they can fight it's (unlimited) tentacles/seaweed fronds, each one is stated as a creature (maybe use giant constrictor snake). Have several attack the party, whilst others fight the crew, pull down masts and stuff in the background. Once the player defeat their tentacles the captain shouts and order - man the oars, take the wheel, or jury-rig the sail - as suits the type of ship. This involves another skirmish and a skill check. When the party succeed the ship breaks free and can escape. However, the angry sea monster remains. It's big enough that the party can see it from miles away, so they know it hasn't gone. Even if the players want to try and take it on, the ship captain and crew refuse to go to sea again until it has gone.