Editing Age of Sigmar/Tactics/Order/Idoneth Deepkin (section)

===Lethisian Defenders===
From the Forbidden Power expansion, this odd army coalition of Idoneth, Stormcast, Kharadron, Fyreslayers, and a random Excelsior Warpriest (w/Gryph-Hound) offers you new Allegiance Ability synergies on a level somewhere between those of the regular faction-only perks and the super basic list for Grand Alliances. By choosing this allegiance, all of your units gain the keyword LETHISIAN DEFENDER and gain access to a couple unique Battle Traits, a Command Ability, 3 Command Traits, 3 Artefacts of Power, and 3 Prayers (arguably the focal point of the Allegiance). Unlike simply taking allies in an otherwise Idoneth list, which is limited by the 1:4 unit ratio rule and (in Pitched Battles) the ≤ 20% pts rule, this allegiance allows you to take basically whatever you want from the four armies (two of which you previously had no access to without pulling back to basic Grand Alliance: Order), with the following exceptions:

*no Hammers of Sigmar (most Stormcast named heroes) [notable exception being the Celestant-Prime]
*no Vostarg units (Fyreslayers) [but Kharadron named characters are legal]
*not Volturnos [though you can take Lotann]
*Battleline is restricted to the armies' vanilla offerings (Arkanaut Company, Liberators, Vulkite Berzerkers, and Thralls) since you can't take the other options without allegiance to the specific factions). One notable exception here that might (but hopefully won't) be errata'ed is that Kharadron Endrinmasters (w/Dirigible Suits) as your general can currently grant you access to Skywardens and Endrinriggers as Battleline w/o KO allegiance, allowing you the potential to make an all-flying army with balloon-boiz and your Akhelains/Eidolons (plus whatever scattered Stormcast units you like).

Otherwise, all other units from the four armies are legal, though of course it makes sense to lean in on priests since all priests in your army gain a free prayer that, like most prayers in the game, goes off on a 3+ and can't be unbound. Also, one of the Command Abilities makes your non-priest general a priest as well, or gives a priest-general access to another prayer. Currently, in addition to the Excelsior Warpriest (a fantastically efficient option with a solid prayer of his own and able to unbind one spell per enemy hero phase), the four armies offer us access to the following priests:

*Stormcast: Lord-Relictor, Lord-Veritant (the superior option by far, super anti-Wizard)
*Fyreslayers: Auric Runemaster, Auric Runesmiter (able to deepstrike 1 Fyreslayers unit), Auric Runesmither on Magamadroth
*Idoneth: Isharann Soulscryer (notably lacking a prayer of his own)
*Kharadron: unfortunately no priests (though an Aetheric Navigator really should be)

The Battle Traits for the Army are as follows:
*all units can re-roll Battleshock tests (sadly mostly redundant for all Idoneth squads except Allopexes)
*all Human and Duardin units get +1 to-Hit in melee against enemies that charged them (on that turn)
*all Akhelian units get +1" Move and +1 on charge rolls, but sadly doesn't help non-Akhelians
*Command Ability: only works for Liberators wholly within 12" of a Stormcast Hero (gives them +1 Save in exchange for not being able to pile-in) [meh]

Drawbacks:
*you no longer benefit from any of the army's Allegiance-specific synergies (most-notably here, Tides of Death and Forgotten Nightmares), but you get to have fun allying with Duardin, more Stormcast, and have some Priestly fun!
*there's no specific lore to the army (besides the prayers), but if you want Wizards, you can always take Endless Spells or use the one Realm Spell from GHB20 (same goes for one extra artefact)
*unfortunately, there's no fix if you don't like the 3 Command Traits
*no new Warscroll Battalions, but you're allowed to use any of the existing ones

Ultimately, though most-likely not powerful enough to be competitive, I hope that GW will make more coalition allegiances like this, as it not only mixes things up, but it creates opportunities for you to collect and learn other armies, and it's super fluffy lore-wise. Plus, the Grand Alliances really don't have enough going for them, so this kinda makes up for that, meeting halfway between.