Underwater adventure ideas needed

[LuciantheLight]

So my party is about 2 months away from ending a large chapter in our campaign and I've been thinking about the follow-up.
Eventually I came up with the Idea of doing an underwater adventure though at the moment I'm a bit stuck :/

What I've already come up with.

The BBEG will be some Cthulhu-esque offspring. This fits nicely since the final enemy will be a great old one.
Each player will get some sort of magical amulet which allows them to act like whales and last long with just a bit of air.
Since they still need to breathe I thought to include some form of kelp which traps air.
I want the adventure to be scary, the party alone in the cold, dark depts. Able to hear leviathans and other large aquatic predators but not able to see it. Having to search of oxygen just to survive.
Fire spells would do limited damage while thunder/ lightning based spells will do AOE damage.


So my question for you guys is are there any other mechanics I could use? Or perhaps some interesting story elements?
Are there any source book on the subject?

[QuickLyRaiNbow]

So my party is about 2 months away from ending a large chapter in our campaign and I've been thinking about the follow-up.
Eventually I came up with the Idea of doing an underwater adventure though at the moment I'm a bit stuck :/

What I've already come up with.

The BBEG will be some Cthulhu-esque offspring. This fits nicely since the final enemy will be a great old one.
Each player will get some sort of magical amulet which allows them to act like whales and last long with just a bit of air.
Since they still need to breathe I thought to include some form of kelp which traps air.
I want the adventure to be scary, the party alone in the cold, dark depts. Able to hear leviathans and other large aquatic predators but not able to see it. Having to search of oxygen just to survive.
Fire spells would do limited damage while thunder/ lightning based spells will do AOE damage.


So my question for you guys is are there any other mechanics I could use? Or perhaps some interesting story elements?
Are there any source book on the subject?

The best Cthulhu-esque monster is the aboleth, which is CR7. I don't know what type of level range you're looking at, but an aboleth with class levels would make a great final boss. As written it's really not that far off Dagon from Shadow Over Innsmouth. There are a number of decent enough aquatic monsters, whether your party fights them or not. I've liked the 5E dragon turtle for a while and wanted to use one.

I'd recommend watching Blue Planet, especially the deep ocean episodes. The sea is a weird, weird place and if we knew everything there was to know about it none of us would ever get near it again. Stormwrack is a 3.5E sourcebook that might have some information, though it's more focused on seafaring.

The biggest piece of advice I can give you is to think really, really hard about whether this is something you want to do. Underwater adventures always seem cool but often turn into massive pains in the ass. They are very, very difficult to run in ways that are fun rather than frustrating. If you're interested in running an underwater adventure because it's underwater, I'd strongly advise you to think about the story you want to tell and the experience you want to create, and about alternate ways to do that.

[Aett_Thorn]

Remember to keep in mind that being underwater basically allows for movement/combat in three dimensions at all times. Movement on something like a grid for combat is therefore going to get a bit tricky.

However, this does allow you to do some fun things, such as attack the party from above/below instead of just from the front/back/sides.

I would also allow for the party to find some weird spells on scrolls underwater. Reversals of spells that are used above-water, for example. While above-water, you have spells to create water, wall of water, watery sphere, etc. Below the surface, you might have similar spells such as create air, wall of air, and airy sphere. What these do is up to you, but facing them for the first time when fighting a spellcaster that is used to fighting underwater may make for some interesting fights.

For instance, a Wall of Air might make anyone passing through it make a Str check or they fall to the bottom of the wall (basically you're removing buoyancy in an area). Airy Sphere might not do much to 'normal' creatures that are used to breathing air, but could be useful in harming creatures that can only breathe water.

[Bloodcloud]

You know how in every video game the water level is the worst?

Are you sure you want to feature a water level?

Besides, underwater becomes quickly a non-issue if the player choose the right class/race, or is overwhelmingly crippling. Expect lizardfolks, sea elfs, and water genasi to thrive, warlock has an invocation for unlimited water breathing, moondruid will be OP by level 4, and by level 5 water walk and water breathing rituals come online. Those spells are available for druids, tomelocks, and wizard gets water breathing but not walk i think.

So just warning you, such a campaign might get real hard to balance by level 4.

[N810]

You know how in every video game the water level is the worst?

Are you sure you want to feature a water level?

Besides, underwater becomes quickly a non-issue if the player choose the right class/race, or is overwhelmingly crippling. Expect lizardfolks, sea elfs, and water genasi to thrive, warlock has an invocation for unlimited water breathing, moondruid will be OP by level 4, and by level 5 water walk and water breathing rituals come online. Those spells are available for druids, tomelocks, and wizard gets water breathing but not walk i think.

So just warning you, such a campaign might get real hard to balance by level 4.

Other possible aquatic races: Darfellans, Bullywugs, aventi, & Triton.

[Unoriginal]

If you really want a very high-CR Chtulhu-spawn BBEG, then a Kraken or an Empyrean modified to have an aquatic theme would be where it's at.

I would advise against modifying spells. 5e spells work the same underwater than inside it.

Another advice I can give you is to only do some part of the adventure underwater, unless you want a full party of aquatic adventurers (which is legitimate, but not what everyone would sign up for).

[the_brazenburn]

Check out the 3.5 splatbook Stormwrack for inspiration. It's not a great book, but there's at least some decent ideas.

For the BBEG, a kraken is probably the most powerful aquatic Cthulu-esque in 5e. Another option, which I used in an aquatic dungeon of my own, is some sort of demented, infernal shark created by a group of sahuagin priests of Sekolah. Exploring the wreckage of the sunken temple to the heathen shark-god, while being hunted by a 30 foot, snaggle-toothed shark and a pack of sahuagin, is about the creepiest thing I can think of.

[iTreeby]

Airy Sphere might not do much to 'normal' creatures that are used to breathing air, but could be useful in harming creatures that can only breathe water.
I would run airy sphere like banish or reverse gravity, you are put in a bubble that is rushing towards the surface, after you pop it or fight your way out you can swim back. The spell could be used to escape quickly or remove an enemy from a battle.

You can also use pressure changes to cause damage from the bends.

[N810]

Maybe ...

-1 to hit for blunt weapons
and
+1 to hit for piercing weapons

due to being under water.

[Demonslayer666]

See Chapter 9 in the PHB, Underwater Combat, page 198.

Water Breathing is a great spell for adventuring under water. It lasts 24 hours and can affect up to 10 people. Ring of Swimming is also very handy.

I would not allow casting spells with verbal components unless they were able to talk freely under water (breath water, or in an air bubble of some sort). I would also require a concentration check for somatic components unless they had a swim speed (DC 10).

I also limit the amount of gear you can swim with to strength score x2 (normal swimming, not magical). You can push it to Str x3, but that requires an athletics check.

[Longman]

Get a cave system map and flood it?

There are massive underwater caves out there for an aboleth to lurk in. Or a morkoth. You just have to sail out to the continental shelf and then swim down and go inside!