"Marge. Kids. Everything's gonna be just fine. Now go upstairs and pack your bags. We're gonna start a new life... under the sea."
- – Homer Simpson - "Homer Badman".
"I got no soul, but I am a soldier"
- – The Killers - "All These Things That I've Done". Well, that's how we remember it going.
"We have been hiding long enough. The time has come for Atlantis to rise again."
- – Orm
The Idoneth Deepkin are fucking awesome. They are fish elves who ride giant fish into battle, what’s not to love?
The Idoneth Deepkin are a new faction of Warhammer: Age of Sigmar and is composed of marine aelves and asorted sea fauna. As the previous writer said, they are rather awesome in their uncommon design and the creatures they bring. Also, elves riding sharks and eels. Yeah, sweet looking.
After Teclis, Tyrion, Malerion, and Morathi made Slaanesh start shitting out elf souls, Teclis took the devout of Mathlann (the deceased elf god of the ocean) and taught these newborn aelves, the "Cythai", about the old world and their gods. However, they weren't exactly model elves: withdrawn, resentful and traumatised by their time within Slaanesh, Teclis wasn't thrilled with them. He tried to find what went wrong, but the Cythai weren't cooperative (Teclis' methods made some fall into madness, so they weren't exactly without reason) and fled into the oceans of the Mortal Realms. Teclis, being a dick tried to exterminate them for good measure, but his brother Tyrion convinced him of letting them go.
The elves, now the ‘Idoneth’, didn't learn that Tyrion had made Teclis cool down and stayed in hiding. They quickly found that their souls were weak and only one in a hundred survived past infancy. It took a while but eventually the Idoneth found that if you kill someone and put their soul into an elf, they won't die prematurely (but depending of the origin of the soul, this only delays the unavoidable). So they started to do exactly that to other people. At first it was only for survival, but later they did it for the expansion of their newly found enclaves. At first it was only one in Hysh, but after the discovery of the Whirlways, whirlpools that work as underwater Realmgates, they started to get into the rest of the realms. Due to differences among the Cytharai and a growing population, they expanded to all Mortal Realms save Azyr and founded several enclaves in all of them.
The Idoneth becames quite the hikkikomori: betrayed by their creator, forced into bad habits for survival and hated by many. They thought that they didn't need the surface world and limited themselves to the oceans, leaving only for raiding souls. However, not even their swift raids, their memory-changing magics or their ruthless "leave no witnesses" policy fooled everyone. Sigmar suspected that something dangerous was in the oceans, while Alarielle found them and outright went to battle against them until the Deepkin in Ghyran offered a truce as terms of surrender. After torturing a Keeper of Secrets whose warband was destroyed by them into spilling the beans, Archaon confirmed his suspicions that there were more aelves than would seem.
They also have kind of a big issue with Nagash. You see, Nagash knew about the disappeared souls and is kind of territorial in regards to the dead. So, because they're stealing souls to save themselves, he's not happy with them (read: really hates these guys). At first he had no idea what was taking them or where, despite extensive searching on his part. After the accidental drainage of a sea in Shyish (How the hell do you drain a sea "accidentally"?!) by the Skaven (...oh, THAT's how. Even better, everyone's favorite Grey Seer was responsible), Nagash caught on to their existence and location. Now he’s emptying an entire ocean in Shyish and sending waves of undead after them in all the realms just to make sure he can stamp them out because he’s the only guy who believes himself to be allowed to have dead souls.
The Idoneth society is divided in three clear castes. First we have the Namarti, composed of those 90% of Idoneth who were born wth imcomplete souls, extremely pale skin, short lifespan (for an aelf anyway, they may still live more than us puny humans) and without eyes that are the majority of the population and are the workforce. Then we have the lucky 10% dudes that were born with complete souls (aka, average aelves): the Akhelians, the warrior caste and those who ride the sweet-looking eels, sharks, Deepmares and Leviadons; and the Isharann, that are the magic users and priests. However, the first ones among all of them were the Cytharai which due to accidents, wars or other shenanigans all of them have died out save the exception of the High King of the Deep Volturnos, who is still alive after millennia and a being a fighter who leads from the front.
One of the more weird phenomena that happens around these guys on land is the so-called Ethersea. This is a manifestation of their marine magic, taking the form of a mist that enables them to use their superior sea-faring abilities and allow their sea beasts to survive where there's no water in miles and move as if they were in water. The Ethersea also have the secondary effect that it manipulates the land and makes it gain deep-sea characteristics spontaneously, like shipwhrecks, coral, fish shoals to start running freely, bubble breath...
One of the few times Mantic served as precursors
Amazingly gw favourite copycats Mantic games started making a raange of seaborn elves before GW for the kings of war game in the form of The Trident Realm of Neiritica. Which has more sea people and less of an elfin look, but it still feels like a similar army to the Idoneth deepkin.