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During the last days of the World-that-was, Slaanesh glutted himself on countless aelven souls. The last he consumed were those who worshipped Mathlann, for they had hidden themselves well from his sight.
During the Age of Myth, the new aelven gods Tyrion, Teclis and Malerion, aided by Morathi's knowledge, captured Slaanesh and began to extract the half-devoured souls from his stomach. The children of Mathlann emerged first, then Teclis claimed their souls as his own and remade them as the Cythai- the Awakened. Teclis wished to make the Cythai the greatest of the aelven races, the most noble and pure of their kind. In Hysh, he built a great city for them and taught them powerful magic. Yet something was amiss, for his creations grew fractious and withdrawn, becoming resentful to his tutelage. Teclis gazed into their souls and instead of light he found a shadow that even he could not penetrate. Weaving great spells, Teclis shone a bright light of truth upon the Cythai to reveal their secrets. Some collapsed into madness, the rest - fearful for their lives - fled to the sea.
They named themselves Idoneth (a name meaning "extreme seclusion") and hid deep beneath the oceans, protected by arcane spells and far from the temptation of the realms, then they began to form new civilizations. Yet the Idoneth had suspected their souls were contaminated. When the first aelf children were born the curse was realised: barely one in every hundred newborn aelves survived past infancy. Those with the spirit-sight could see their progeny were born with swiftly withering souls; as a consequence their lives were cut tragically short. The Idoneth desperately sought a cure as their race declined and, in time, they found one. By capturing the soul essence of another creature, they could sustain the lives of these Namarti, if only for a short while. Those among the Idoneth born with souls - the Isharann and Akhelian - looked at their kin and wept. The survival of their race depended on the souls of others.
Over time the Idoneth tried many arcane ways to sustain, cure or heal those damned with withering souls, and began experimenting with drawing the souls from deep sea beasts and impanting them in the afflicted but quickly discoverd that these would only last days. Human, orruk , duardin or sylvaneth souls proved more powerful but it still took as many as half-dozen souls to give a deepkin even a third of their normal life.[5b]
The Enclaves or nations of the Idoneth are hidden in the darkest depths of the larget oceans of the Mortal Realms. Connected by underwater Realmgates called whirlways, the Idoneth are able to support each other in times of dire need and launch raids across the realms.
The first city of the Idoneth was named Gealrachi, and was hidden beneath the waves of the Gealus Ocean in Hysh. Yet the Idoneth were ever a fractious race and the discovery of the whirlways saw them split into six enclaves and disperse across the realms to lead their own isolationist existences. The noblest of the Idoneth were called Ionrach and travelled to Ghyran to establish their own city, while the aggressive and impetuous Fuethán found themselves drawn to the boiling seas of Aqshy. 
- Briomdar: Once part of the Ionrach they inhabit the Green Gulch in the Realm of Life, from where they raid for souls.[5i]
- Dhom-hain: Thye parted ways with thir Ionrach kin in Hysh and settled in Ghur, establishing Rúndhar in the Black Trough in the Ominod Sea. They are renowned as savage fightrs.[5i]
- Fuethán: Considered the most impulsive of the Cythai, they found a new home in the Mordacious Sound.[5i]
- Ionrach: First of the Idoneth who they travelled to Ghyran, where they established the city of Príom beneath the Maithnar Sea.[5i]
- Mor'phann: The most insular of the Deepkin who reside at the base of the fog shrouded sea known as the Great Quagmire in the Realm of Shyish.[5i]
- Nautilar: After internal strife in Ionrach, nearly a quarter of their people founded a new city on the back of a Great Scaphoden, travelling across the Realms.[5i]
Other known Enclaves
- Aighmar: a Mor'phann descendent enclave,[5i] that during the Age of Sigmar was discovered by Skaven which burrowing between realms. Aigmar was breached and ultimately annihilated by Skavens armies.[5f]
- Ilmeth: They can be found at the bottom of the Lampleus Sea.[5i]
- Khaphtar: In the Time of Tribulations they appealed for help from King Glorian of Laebrea
- Laebrea: Ruled by King Glorian in the Realm of Ghur
- Morladron: In the Realm of Ulgu.[5i]
- Motlynian They inhabit the Barricadius Reef.[5i]
- Ymmerloc: dwell in the Helfrost.[5i]
The enclaves of the Idoneth are split into three castes:
- Namarti: the most numerous part of the Idoneth Deepkin.
- Akhelian: the martial caste.
- Isharann: the masters of magic.
Each enclave has a ruling council made up of Isharann and Akhelian. Between them they decide the path their enclave will follow, be it aggressive expansion, total isolationism or somewhere in between. It is a harmonious relationship for the most part with no overall ruler because the Isharann know they cannot claim new souls without the Akhelian caste leading their raids. In return, the Akhelians know they can't do much with their captured souls without the arcane knowledge of the Isharann.
Most denizens of the Mortal Realms know little or nothing of the Idoneth Deepkin, for they have been fastidious in their isolation, cutting themselves off from the realms and shrouding their activities in arcane rituals and dark magic. A survivor of an Idoneth attack may have fleeting memories of them, but these will soon fade into half-remembered nightmares as the spells of the Tidecasters take hold. Yet the Idoneth's continual need for souls has finally etched their existence in the minds of several races, most of which now seek revenge on the aelves that have plagued their land and slain their peoples for centuries. The Idoneth can't make amends easily, for their curse can't be cured and souls must be harvested, yet several enclaves have pledged their allegiance to Sigmar and the forces of Order, promising to fight against he growing forces of Chaos wherever they can.
- Allopex - A raiding beast.[5e]
- Deepmare - raiding beast of Idoneth leaders.
- Druilfish - A Rock-boreing fish used for expanding the networks of undersea cities and to carve out underhalls[5e]
- Fangmora Eel - A raiding beast.[5e]
- Fuiadon - The molten discharges of this creature are used to fire the forges.[5e]
- Gilotán - Mainly used by Fuethán enclave, this spiny beast can unleash flame at its enemies.[5h]
- Great Scaphodon - A continent-sized beast, they live in Ghur and wanders into seabeds. The main city of Nautilar is located here.[5f]
- Leviadon - A great draft beast and living fortress of Idoneth phalanxes.[5e]
- Luminar Fish - Their bioluminescent light is used to brighten even the deepest and largest abyssal vaults.[5e]
- Ochtar - A tentacled familiar.
- Rakerdart - Used to help to Soulrenders to catch souls.[5k]
- Scryfish - Shoals of these fish attack foes of Soulscryers with the spikes that jut out from their jaws.[5k]
- Stórá - The arcane charge of this creature is used to power many of the fanes of the Isharann.[5e]
Known Wild beasts
- Ankusha – Shell Spines of this creature are near-unbreakable so are used by Idoneth Deepkin raiders to additionally goad their ferocious beasts in battle.[5l]
- Augury Shells – Augury shells come from the Cerithium Sea. They enable the Idoneth spellcaster to better predict course of destiny.[5o]
- Brain Barnacles – These nightmarish parasites native to the ethersea are microscopic arthropods which can attach themselves to brain tissue of sentient creatures and feed off their thoughts. Idoneth harvest this barnacles, capture them in vials and release them toward a most powerful enemies in battle.[5n]
- Brón – The conches of this creature contain in them the lamentation of a Lliandra, Awakened and great sorceress. When shattered the woe of Lliandra is released filling Idoneth around with the stoic and sad grimness. [5m]
- Carcinclaws - Big beasts with spiked armour so thick they are all but invulnerable, and with serrated pincers which can wasily cut aelf in two; native to the Realm of Ghur.[5i]
- Dritchleech - These incredibly rare worms feed upon magic so could drain even the most potent arcane energies. The Idoneth themselves long ago gain an immunity to these worms effects but use them on the battlefield to suppress magic of enemy spellcasters.[5n]
- Gargadon - A colossal monstrosity.[5c]
- Ghurish Kraken – Native to the Realm of Ghur as stated from its name, this kraken have sword-length tooths. Idoneth Deepkin spellcasters can use one of such tooth to create a manifestation of slavering maw of this dreaded creature that can swallow enemies whole.[5o]
- Gnarcuda - A predator native to Realm of Ghur that attack in wolf packs.[5i]
- Gyrfins - A predator native to Realm of Ghur.[5i].
- Kalypsar - Idoneth hunt these beasts[5c].
- Ketus-Shark – From the blood of this fish Idoneth Deepkin make a ‘’’Potion of Hateful Frenzy’’’ which can fill the drinker with berserker fury. After some period though the effect of potion wears off and the drinker became exhausted.[5l]
- Kharibdyss - Idoneth hunt these beasts.[5c]
- Krakigon - Idoneth hunt these beasts.[5c] Krakigon have a natural pigment for concealing its location when need to withdraw. Idoneth Deepkin use this pigment containing it in special vials. When such vial is shattered, the bearer is swiftly hidden in a darkness cload.[5m]
- Living Coral - Corals that Idoneth Deepkin used for creating towers and citadels of their cities; native to Realm of Ghur.[5i] Chorrileums - the great reef-conglomerates that contains all souls of the dead Idoneth of a nearby city also made from this corals.[5j]
- Lurkinarth - Idoneth hunt these beasts.[5c]
- Sithilopod - A thousand-tentacled monster.[5c]
- Terrornight Medusa – From the tentacles of this creature Idoneth distill a potent venom. The smallest cut of the weapon coated with this venom fill the victim with the feeling of approaching doom and cause a severe pain.[5m]
- Voracuda - Swarms of these creatures carry out attacks on Idoneth strongholds.
Known Underwater Races
- Kelpdar - afraid of Idoneth and everything that connected with them[5c]
- Merwynn - afraid of Idoneth and everything that connected with them[5c]
|Aemorthis||Idoneth Deepkin||Akhelian King||Aelf||In the Time of Tribulations he saved a contingent of Stormcast from a horde of Deathrattle skeletons raised by necromancers.|
|Gwyth Banrionic||Idoneth Deepkin||Akhelian King||Aelf||In the Age of Myth she was the first and perhaps the greatest queen of the Dhom-hain Enclave..|
|Glorian||Idoneth Deepkin||Akhelian King||Aelf||In the Time of Tribulations he lured a large force of Ironjawz into a Realmgate to Shyish.|
|Lotann||Idoneth Deepkin||Isharann Soulscryer||Aelf||The Warden of the Soul Ledgers|
|Radharcith||Idoneth Deepkin||Isharann Soulscryer||Aelf||He led a force that, in alliance with the Sylvaneth defeated a large Chaos force.|
|Volturnos||Idoneth Deepkin||High King||Aelf||The only surviving Cythai and regarding by many Deepkin as their greatest warrior, earning his reputation in the Age of Myth.|
|Akhelian Allopex||Idoneth Deepkin||Order||Warscroll|
|Akhelian Ishlaen Guard||Idoneth Deepkin||Order||Warscroll|
|Akhelian King||Idoneth Deepkin||Order||Warscroll|
|Akhelian Leviadon||Idoneth Deepkin||Order||Warscroll|
|Akhelian Morrsarr Guard||Idoneth Deepkin||Order||Warscroll|
|Eidolon of Mathlann, Aspect of the Sea||Idoneth Deepkin||Order||Warscroll|
|Eidolon of Mathlann, Aspect of the Storm||Idoneth Deepkin||Order||Warscroll|
|Isharaan Soulrender||Idoneth Deepkin||Order||Warscroll|
|Isharann Soulscryer||Idoneth Deepkin||Order||Warscroll|
|Isharann Tidecaster||Idoneth Deepkin||Order||Warscroll|
|Lotann, Warden of the Soul Ledgers||Idoneth Deepkin||Order||Warscroll|
|Namarti Thrall||Idoneth Deepkin||Order||Warscroll|
|Namarti Reaver||Idoneth Deepkin||Order||Warscroll|
|Volturnos, the High King||Idoneth Deepkin||Order||Warscroll|
- 1: White Dwarf April 2018, pgs.32-39
- 2: Battletome: Daughters of Khaine
- 3: Descent - story from the Malign Portents campaign (last accessed 17 April 2018)
- 4: Lotann on the GW Website (last accessed 18 April 2018)
- 5: Battletome: Idoneth Deepkin
- 5a: A Strange Genesis, pgs. 8-9
- 5b: The Great Emptiness, pgs. 10-11
- 5c: The Deep Places, pgs. 12-13
- 5d: The Unseen Menace, pgs. 14-15
- 5e: The Idoneth Deepkin at War, pgs. 16-19
- 5f: Underwater Empires, pgs. 20-23
- 5g: Isharann & Akhelian, pgs. 24-25
- 5h: Idoneth Military Structure, pgs. 26-27
- 5i: The Idoneth Encalves, pgs. 28-33
- 5j: Eidolons of Mathlann, pg. 38
- 5k: Isharan Agents, pg. 43
- 5l: Akhellan Artefacts, pg. 88
- 5m: Idoneth Artefacts, pg. 89
- 5n: Isharann Artefacts, pg. 90
- 5o: Arcane Artefacts, pg. 91
- 6: GW site, Akhelian King (last accessed 22 April 2018)
- 7: As Above, So Below short story on the Malign Portents website