Theory of Games
Storied Gamist
Great thread and great ideas here 

Some of my ideas for the metallurgy issues:How do they control heat or fire? All technology beyond stone and bone needs heat or fire. To smelt metals, forge metals or distil elixirs. How do they control heat underwater with any degree of efficiency and without scalding themselves?
Or do they use some kind of magical biotech? In that, all their tools are creatures designed for the role, or the waste products of creatures designed to produce that.
I think that is where I would start.
Huh. That gave me a flashback to the Secret History of the Dark Sun setting. At one point, the planet was mostly covered in water, and the dominant species (and only sapient one, except for semi-sapient precursors to the Thri-Kreen) were something close to halflings. These halflings didn't have any arcane magic, and only limited elemental (priestly) magic, but they were amazing at "lifeshaping" – essentially biotech. That could be a cool thing for an underwater empire to use.How do they control heat or fire? All technology beyond stone and bone needs heat or fire. To smelt metals, forge metals or distil elixirs. How do they control heat underwater with any degree of efficiency and without scalding themselves?
Or do they use some kind of magical biotech? In that, all their tools are creatures designed for the role, or the waste products of creatures designed to produce that.
I think that is where I would start.
The water around such vents is usually superhot (well over 100 dec C) but not boiling because of the pressure. So how they manipulate and how to the see what they are doing? Are they immune to the heat?Some of my ideas for the metallurgy issues:
- They use underwater volcanic vents
Also, cool but how do they prevent the surrounding water from transporting the heat away and killing the process?
- They use some chemical/alchemical processes to forge metals.
So unique spells?
- Magic, especially Conjuration and Transmutation spells
See my comments on chemistry/alchemy
- They bind Fire Elementals for forges
A bit boring
- They trade for crafted metallic items
- They have colonies or workshops on the surface.
This could be really cool, though perhaps the most work.
- Biotech or Life-shaping (such as what they used in the Athasian Blue Age)
There probably aren't that many sea creatures that have resistance to fire damage, but it's probably done by someone who is resistant to it.Some interesting ideas here but pick one and work out the implications. For instance:
The water around such vents is usually superhot (well over 100 dec C) but not boiling because of the pressure. So how they manipulate and how to the see what they are doing? Are they immune to the heat?
An alchemical process might not involve heat, if alchemy could transmute one material into another (like most infamously lead into gold), then maybe it temporarily converts metal into something that's malleable.Also, cool but how do they prevent the surrounding water from transporting the heat away and killing the process?
Fabricate is a spell that comes to mind.So unique spells?
One of my ideas is that the Empire is a multi-species Empire, I do see some species like Locathah being more suited to living deeper than Sea Elves or Merfolk, and possibly Koalinths being a lot of the soldiers of the Empire given how militaristic their surface brethen Hobgoblins happen to be. So they would have some deep sea soldiers, which might include some of the more elemental water type of beings.My main question is how? How did the empire get that big? The biggest contiguous land empire in history, the Mongol Empire, didn't even conquer a full continent. How on Earth did a single empire conquer all of the oceans on the planet? Especially given the fact that oceans are 3D. You can swim up and down. There's life living at all levels of the ocean, and there would likely be aquatic fantasy races living at all depths. Can whichever species that formed this empire survive at all depths of the ocean, from the world's equivalent of the Mariana Trench to the surface? Let's say the Kuo-Toa and Aboleths live at the very bottom of the ocean. How does the empire make sure they're loyal and don't declare independence? Can their average soldier survive living at the bottom of the ocean for extended periods of time? Or do they have divisions of soldiers that specialize in deep sea diving?
Definitely magic to keep their empire in communication. Teleportation, gateways and planar shortcuts for some of the cohesion of the Empire. I guess parts of the Empire might have to go tolerate going through the definitely independent City of Glass on the Elemental Plane of Water. Which might lead to conflicts with Marids.And not just how did it form, how is it still around? That empire would be over-extended to Baator and back. If the empire has a capital city, how does it tell people on the other side of the planet, which they supposedly rule over, what to do? How do they enforce their laws and policies? How do they collect taxes/tribute? How does the empire protect trade, communication, and travel throughout the empire? How does the empire make sure that not only is it producing enough food, but that it's making specifically enough food for all of the different species in the empire and is delivering the food to the correct locations in an orderly fashion? There would definitely be magic that could help, such as teleportation circles and the Sending spell, but the empire would have to have pretty widespread magic for them to rely on those spells to administrate the empire.
I think there would be factions of the major populations like Sea Elves, Merfolk, Locathah, Koalinths, Tritons, Water Genasi and then the others. Or maybe they aren't divided along those lines. The rulers could be influential members of those species perhaps an elected council, but the rulers could also be Storm Giants or an Empyrean or someone that's secretly a Kraken.What are the internal factions in the empire? Who is in charge of it? Does the empire have strict succession laws to determine who can rule? For example, maybe all monarchs so far have been Storm Giants of a specific dynasty. Or can anyone become emperor if designated as the successor or they gain enough influence/power (be it through military accomplishments, climbing the bureaucratic ladder, or governing a province in the empire)? In the latter case, maybe the empire has near-constant civil wars to determine who is in charge. If they don't have a single ruler, are they a republic? How do they do elections? Who is allowed to vote? How do they do censuses in an underwater environment, where groups of people will likely be moving around constantly?
Sahuagin, Ixitxachitl and Kuo Toans might be the outsiders to the Empire. And with oceans being as large as they are, there's probably a lot of semi-independent client states. But I certainly see the empire as having great underwater cities.Does the empire completely control and administer all regions of the oceans, or are there semi-independent client states in regions that are too difficult to control (deep sea, lakes, underwater caves, etc). These parts could be nominally part of the empire, but mostly self-administering. Similarly, if there are nomadic tribes of hunter gatherers, there's not much point in keeping record of their location and numbers, as they won't be paying much in taxes.
Many of the empire eat seaweed, kelp and shellfish and other things as well as fish.How much of the society is nomadic and how much is settled? In an underwater environment, nomadic cultures would be common as hunters follow schools of fish and other prey. How does agriculture work?
In keeping with the main people being Sea Elves, Merfolk, Locathah, Koalinths, Tritons and Water Genasi, they're at least the middle class in the empire's society. Merrow might be the enforcers kept under control of the others. While powerful being like Storm Giants, Marid and Morkoths might be the privileged.Who are the privileged and marginalized peoples in the empire? How are the privileged rewarded by society and how are the marginalized punished? Does slavery exist and in what form?
They might be threats to the empire, or things they can tame, or the secret rulers.How does the military work? Is the empire attempting to expand onto land and/or into the rivers and lakes of the world, or are they prioritizing keeping the empire together as is? How do they deal with dangerous megafauna, like Krakens and Sea Serpents?
Haven't really thought of the religion, but likely would be Istishia the Greater God (or Primordial) of Water.What is the religion of the ruling class? Do they mostly tolerate other faiths, or do they enforce it as a state faith? If they allow other religions to exist, do they do the Greek/Roman practice of interpretatio graeca/romana of conflating similar deities of different pantheons together as a way of assimilating the conquered peoples? If they forcibly converted everyone to their religion, how does the church work in the empire? Religion is often used by the ruling class to justify their rule (Divine Right of Kings, Mandate of Heaven, etc). There have been heads of state that are also the heads of the religion (the Pope, Islamic Caliphs, the Japanese Emperor in Shintoism, the British Monarchs in Anglicanism). But there are also religious heads that don't have political power. Does the ruling dynasty also serve as the heads of the religion, or is there a religious head that is independent of the state? How much - if at all - do the churches and the empire disagree with each other? Does the empire justify wars of expansion through religion?
Oh, I considered the empire being multi-species as being a given. Empires tend to accumulate more ethnicities as they expand. Unless the empire wiped out the other species as it expanded, it would definitely end up having several different species inside it. My point was to give some thought as to how this would affect the empire, especially with how the depths of the ocean would affect it. Water elementals being in the military is an interesting idea.One of my ideas is that the Empire is a multi-species Empire, I do see some species like Locathah being more suited to living deeper than Sea Elves or Merfolk, and possibly Koalinths being a lot of the soldiers of the Empire given how militaristic their surface brethen Hobgoblins happen to be. So they would have some deep sea soldiers, which might include some of the more elemental water type of beings.
Okay, so how does the empire make sure it always has enough mages casting Sending, Teleportation Circle, and planar gateway spells? The two obvious solutions to me are either powerful magic items or a government-funded arcane university.Definitely magic to keep their empire in communication. Teleportation, gateways and planar shortcuts for some of the cohesion of the Empire. I guess parts of the Empire might have to go tolerate going through the definitely independent City of Glass on the Elemental Plane of Water. Which might lead to conflicts with Marids.
I think there would be factions of the major populations like Sea Elves, Merfolk, Locathah, Koalinths, Tritons, Water Genasi and then the others. Or maybe they aren't divided along those lines. The rulers could be influential members of those species perhaps an elected council, but the rulers could also be Storm Giants or an Empyrean or someone that's secretly a Kraken.
I think organizing them by some sort of caste system would make sense. Maybe with some division based on the region of the ocean they live in. Deep sea species being of a lower caste makes sense to me, given their semi-independent nature and unpleasant habitat.In keeping with the main people being Sea Elves, Merfolk, Locathah, Koalinths, Tritons and Water Genasi, they're at least the middle class in the empire's society. Merrow might be the enforcers kept under control of the others. While powerful being like Storm Giants, Marid and Morkoths might be the privileged.
Definitely. Maybe consider what they produce and trade with the surface. And what they import. The adventurers could even meet an aquatic merchant who has come up to the land to trade.But I certainly see the empire as having great underwater cities.
Maybe they rely on allied aquatic dragons and dragon turtles to fend off other Kaiju-esque threats?They might be threats to the empire, or things they can tame, or the secret rulers.