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

==Allies==

*'''Cities of Sigmar''': They offer the widest variety out of all the allies, particularly the aelven forces.  Here's a more comprehensive description of what works with Idoneth:  
** Word of Pain from a Sorceress helps with -1 to Hit spam in a mage heavy list. (Just remember that they're susceptible to being sniped out).  
** Order Serpentis provide expendable cavalry with good saves to soak up fire for your Akhelians.  A Black Dragon or War Hydra are good choices for fast heavy hitters and/or shooting.  
** While Scourge Privateers would be great thematically, only the Scourgerunner Chariot is really worth considering from this group (and there's a lot of bad blood between the two factions lorewise).
** Shadow Warriors can deepstrike, and (if in cover) outshoot Reavers from afar (though they're less mobile once on the field). 
** Wanderers also offer several benefits (though most of the following description is outdated).  A unit of 20 Glade Guard in your army provides you with a highly reliable "remove lynchpin" button that will likely enrage your opponents.  Sisters of the Thorn are an awesome addition to a Cavalry army and provide a very nice spell to boot.  Eternal Guards re-roll saves of 1 and 2 if they are in cover, are as fast as Namarti Thralls and cost 70 points (currently 130) per 10 models (with a super discount at max size). A Waywatcher has the invisible hunter rule, which gives shooting units that target him -1 to hit, and can be given to other Wanderer units within 18" for 1 command point (combine with Tides of Death to make shooty armies cry).
** Battlemages are excellent support units that can be tailored to your needs, and the Battlemage on Griffon packs a punch too.
** Phoenix Temple are amazing.  Phoenix guard can eat enemy fire with minimal losses given their 4+ save and a 4+ save-after-the-save.  Throw in a Phoenix Guard Anointed and they not only become fearless but the Anointed gives you an extra dispel.  The Flamespyre Phoenix is also a great unit, trading the dispel for dropping MW on enemies and a one-use chance to come back from death on a 4+.
** Ironweld Arsenal Gyrobombers are a good and cheap choice for dropping MW on enemy units with their bombs, but Gyrocopters with Steam Guns are both much faster (better able to keep up with IDK flying units) and have a MUCH more potent close-range, anti-horde missile attack. 
*'''Daughters of Khaine''': The best option by a long shot and currently lore-friendly. What they bring are cheap and expendable units of 90 points per 5 Harpies that can Deep Strike in, and help block shots for the rest of your army via ''Forgotten Nightmares''. 2 Units of Javelin Harpies, or one unit of Sword Harpies is pretty likely to one shot a 5 wound hero to boot. Doomfire Warlocks are also solid allies for a Cavalry army and easily able to keep up as well as providing good shooting and a source of mortal Wounds. Your best bet here is probably a unit of Khinerai, seeing as how they can deepstrike independently from any other unit, and offer a decently strong enough attack to help shift an chaff unit off a back field objective. It doesn't hurt that they are cheap points wise too. The new Khanite shadowstalkers are quite good as allies, being able to teleport wherever they want. For a hundred points they are an interesting 9 man objective grabber, because sometimes your eels can't quite do it alone. 
**'''Alternate Opinion:''' While Daughters of Khaine are indeed an extremely powerful force on their own, they don't make very good allies for a number of reasons. They excel at being an extremely aggressive force when you pile on their faction bonuses (escalating table, 6+FNP, Temple abilities, extra prayers, their NUTS spell lore, not to mention any number of internally consistent abilities and rules) on their own without those they tend towards being overpriced or fragile. While you might get some mileage out of a unit of Khinerai or Doomfire Warlocks, you'd probably be better off with some more Idoneth. 
*'''Lumineth Realm-lords''': The only things worth considering here are Spirits of the Wind (Severith or generic) and Auralan units.  Wardens are better defensively than Namarti-Thralls with Shining company and a superior save Sentinels offer better shooting and Dawnriders offer cheap chaff removal.  Also, all three Auralan units are wizards and give the Idoneth a greatly needed hotbed of MW dealers.
*'''Stormcast Eternals''': For a Namarti-heavy army, Liberators or Sequitors make fantastic cannon fodder when combined with a Lord-Castellant and ''Forgotten Nightmares''. Other than that, Vanguard-Raptors make for some nice budget shooting. Fulminators are a nasty addition to any Cavalry heavy army, and combined with ''Tides of Death'', will get your opponent to flip the table fast. A Knight Azyros can give your units a reroll 1 to Hit for enemies in 10". Also, a Knight Vexillor might use his redeploy ability to make your Stormcasts keep up with your Idoneths. A Celestant-Prime waiting in the heavens to boost his Attacks characteristic and then dropping in on turn 3 in High Tide (if Ionrach) can do all kinds of work. Turn 2's not bad either so that you can take advantage of another Cometstrike. 
*'''Sylvaneth''': The only things worth considering here are Kurnoth Hunters for some additional shooting and Tree Revenants for Warmachine hunting without investing hundreds of points into a Soulscryer and an escort.  Additionally, you might want to consider their Outcasts Battalion (3 units of Spite-Revenants), which at 280pt for the minimum-sized battalion, will likely remain the cheapest way to squeeze an extra battalion in. That said, it's currently impossible to field this in a Deepkin list in Vanguard and meet all of the faction-units-per-allies-units requirements, so save this for your Battlehost-and-larger games.
*'''Gotrek:''' Despite costing 520pts, Gotrek can be brought as an ally in any Order faction even if that's more than the usual allies allowance. It's worth mentioning for the Ionrach enclave because he'll get the benefit of the Tides. Gotrek's an absolute beast in melee but is let down by his low movement and the fact he can never be teleported/ambushed onto the board etc.
*'''Bundo Whalebiter''': A Kraken-eater Mega-Gargant mercenary available to all Order factions. Taking him uses up your entire ally points allowance and he ignores any Behemoth or Hero limitations. A big terrifying centerpiece who can kick objectives around and instantly kill models by shoving them into his net. You can choose at the start of the Combat phase for him to fight at the end of the phase, in exchange for re-rolling failed hit rolls. If you want a big monster that isn’t the Turtle, then this is your pick.

===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.