Ruleshammer 40k: The Arks of Omen Detachment and Allies

Welcome to Ruleshammer! This week we’re covering the new Arks of Omen Detachment. Remember the banner below will take you to the Ruleshammer 9th Edition Compedium, for all the questions I’ve answered for the last few months!

The new mission pack is nearly here and with it we have brand new rules for mustering armies, giving us one of the biggest shake ups since the edition began. This new missions pack throws out almost all the detachments of old and replaces them with a shiny new Arks of Omen Detachment (which I will be calling an Arks Detachment, as AoO just looks ridiculous). Previewed recently on Warhammer Community, this detachment offers a lot of flexibility:

Source: Warhammer Community

There’s a lot going on here, and there’s a lot that wasn’t answered by the WarCom article, so today we’re going through the rules and addressing some of the major questions players are likely to have.

Before we dive into these questions we’d like to thank Games Workshop for providing us with a preview copy of the Arks of Omen Grand Tournament rulebook for review purposes.

Can I take the old detachments still?

No, other than a single Allied Patrol. If you’re anything like me, this limitation isn’t actually obvious if you skip straight to the Muster Armies rules in the book. There, the book says that:

…each player must include one and only one Arks of Omen Detachment in their army…

but it doesn’t say you can’t include other detachments in that same paragraph, and even implies the others continue to exist based on the mission packs continued use of the Patrol or Super Heavy Auxiliary Detachments for Allied units. However it is made clear earlier in the book in the Introduction on page 1, after explaining that these detachment are one of the biggest changes the first bullet points of that section ends with:

Players cannot include any other detachments in their army – their army will therefore consist of one compulsory Arks of Omen Detachment, and possibly one optional Allied Detachment.

Games Workshop are really keeping us on our toes with reading comprehension here – no skipping introductions as there might well be rules in them! I’m still waiting for the day when rules are hidden in a paragraph of lore.

Are the Compulsory units in Addition to the other slots?

Yes. Unlike with previous detachments, the compulsory slots for this detachment are a choice and they are in addition to the optional slots. So if you decide to make Elites your compulsory choice you have 6 Elites slots, plus 3 more character-only Elite slots. And then the remaining optional slots as well.

Flyer slots are always limited to two but remember that the Balance Dataslate restricts “Aircraft Models,” not units – so those also limited to two on top of this restriction. So even if your Aircraft come as a unit like Tau Remora drones, the limit is still two models.

Note that Dedicated Transports slots remain tied to your INFANTRY unit count.

Can I take an Allied Detachment of the same faction?

Mostly no. Allied Detachments are a Patrol Detachment of specifically allowed factions depending on the faction keywords of your own force.

The main exception to this are Imperial Knights and Chaos Knights. The former has a special exemption to run an IMPERIUM Ark Detachment of Imperial Knights, and then add an Allied Super Heavy Auxiliary Detachment containing a single Freeblade unit. Likewise, Chaos Knights armies can do the same with Dreadblades.

Some examples of this have been previewed on WarCom, though the full list is much longer and covers more armies:

Source: Warhammer Community

Other than that, notice in these examples that the allied detachment for Genestealer Cults has to be a Brood Brothers detachment (the rules which very recently got updated in their FAQ to work with the new Astra Militarum codex), and the rules for what faction allied detachments can be is specific to the ones listed in the Battle Brothers section.

You can only include an Allied Detachment if its Faction is a permitted Allied Detachment for your Arks of Omen Detachment, as described in the Battle Brothers section on page 8.

How come the Genestealers section mentions Auxiliary Support Detachments?

No idea. The rules for allied detachments are quite specific:

… this must either be a Patrol Detachment or a Super-heavy Auxiliary Detachment… 

but the Genestealers section goes rogue and widens the options to also include an Auxiliary Support Detachment. Not sure why the first section doesn’t mention them being an option, or not mention any at all and let each Battle Brothers section detail the allowed detachments. As it stands though, I’d let the Battle Brothers section win in this case.

What about units that used the Supreme Command detachment or have “Agents of” Rules?

Several key units in a few factions of 40k have the Supreme Commander or Primarch keywords; these have in the past let them make use of the Supreme Command Detachment to join armies without breaking detachment rules for other detachments of different sub-factions, such as taking Shadowsun with a Sa’cea Sept Tau Army. In Arks of Omen games that detachment option is not longer available. I’m not going to cover every possible example of how this changes things but here are a few examples:


While Shadowsun would be excluded from both <Sept> checks and the army wide check for Philosophies of war via the SUMPREME COMMANDER keyword.

If your army is Battle-forged, <SEPT> units in T’AU EMPIRE Detachments gain access to the following sept rules, provided every <SEPT> unit (excluding SUPREME COMMANDER units) in your army is from the same sept. If every <SEPT> unit in a T’AU EMPIRE Detachment is from the same sept, that Detachment is referred to as a Sept Detachment.


If every unit from your army (excluding T’AU AUXILIARY, SUPREME COMMANDER and UNALIGNED units) is from the same Sept, then at the start of the battle, when you have determined who has the first turn, you can select one of the Tactical Philosophies below. T’AU EMPIRE units (excluding T’AU AUXILIARY units) from your army gain the benefits of that Tactical Philosophy. If both players have a similar ability, the player who has the first turn chooses first.

However she does have the TAU SEPT keyword, and the <SEPT> rules section ends with

If your army is Battle-forged, you cannot include units from two different T’au septs in the same Detachment. You can find out more about Battle-forged armies in the Warhammer 40,000 Core Book.

so currently Rules As Written she can only be in a Tau Sept army without making the list invalid for Battleforged.


Abaddon has a few different keywords he can use other than just SUPREME COMMANDER, if he’s the armies Warlord (which he usually has to be) he gains the AGENT OF CHAOS keyword as well. The big problem however is that Abaddon has the BLACK LEGION keyword, and while he gains AGENT OF CHAOS as your Warlord, he’ll still break a detachment that isn’t a Black Legion detachment if he’s included in it – if you put Abaddon into say, an Emperor’s Children Detachment, they still get Legion Traits because he’s got AGENT OF CHAOS, but it’s no longer an Emperor’s Children Detachment, since the Codex doesn’t have a carve-out for that happening and before you solved it by putting him in a Supreme Command detachment.

On that note, removal of the Supreme Command Detachment makes taking Abaddon with CHOAS KNIGHTS far more restricted, limiting you to a single Allied Detachment of Chaos Knights with a CHAOS force. You also can’t just build an Arks Detachment of Abbadon and some Knights either because of the restrictions of that detachment:

Restrictions: All units must be from the same Faction (and that Faction cannot be IMPERIUM, CHAOS, AELDARI, or TYRANIDS). This detachment cannot include any understrength units.

With no exclusion for AGENT OF CHAOS or SUPREME COMMANDER an Arks detachment would need a more specific sub-faction keyword, such as HERETIC ASTARTES, between Abaddon and any Knights he was being run with. This means that Dog Walker lists are dead unless we get an FAQ making it otherwise.

Speaking of HERETIC ASTARTES, he does share that keyword with Thousand Sons and Death Guard, however neither faction have any exclusions for either of Abaddon’s special keywords so they’d lose all of their detachment and army-wide abilities.

There are some Allies options for CHAOS, TRAITORIS ASTARTES, and LEGIONES DAEMONICA that would result in armies with Abaddon but not with an army of Chaos Knights, now it would just one Chaos Knight in a Super Heavy Aux. I recommend a full read through of the Battle Brother section to really explore the options provided.


At time of writing it’s not entirely clear if the Assassins actually have legal 9th edition rules (the supplement with their rules has an expiration date of January 2023). However their most recent rules have this ability:

If your army is Battle-forged, you can include 1 AGENT OF THE IMPERIUM unit in each IMPERIUM (excluding FALLEN) Patrol, Battalion and Brigade Detachment in your army without those units taking up slots in those Detachments. The inclusion of an AGENT OF THE IMPERIUM unit does not prevent other units from their Detachment from benefiting from Detachment abilities (e.g. Chapter Tactics, Defenders of Humanity etc.), and it does not prevent other units from your army benefiting from abilities that require every model in your army to have that ability (e.g. Combat Doctrines). An AGENT OF THE IMPERIUM unit included in a Patrol, Battalion or Brigade Detachment in this manner is ignored for any rules that state all units from that Detachment must have at least one Faction keyword in common (e.g. in a matched play game) and when determining your Army Faction.

The first part is not relevant to this mission pack as an Arks Detachment is not a Patrol, Battalion, or Brigade. However the highlighted section about not preventing army-wide rules is quite important. It means that if you were to take a Assassin in an Allied Auxiliary Support Detachment as allowed in the IMPERIUM Battle Brothers entry, then that inclusion would not prevent the army they join from benefitting from army wide rules such as this one for Space Wolves:

A SPACE WOLVES Detachment is one that only includes models with the SPACE WOLVES keyword (excluding models with the AGENT OF THE IMPERIUM or UNALIGNED keywords). Note that such a Detachment is also an ADEPTUS ASTARTES Detachment, as described in Codex: Space Marines, and as such all the ADEPTUS ASTARTES Detachment abilities also apply to that Detachment.

There’s a few levels to this; first of all the Space Wolves detachment would continue to be one even with Assassin in it, however due to their inability to be in the same detachment now that part doesn’t do much any more. The rule then continues:

    • SWIFTCLAW units in SPACE WOLVES Detachments gain the Headstrong and Berserk Charge abilities.
    • If every unit in your army (excluding UNALIGNED units) has the SPACE WOLVES keyword, then every unit that has the Combat Doctrines ability and is in a SPACE WOLVES Detachment gains the Savage Fury ability, below.

This bullet point is what the Assassin rule lets you ignore, even though every unit in the army doesn’t have SPACE WOLVES they would still get their “super” doctrine Savage Fury even with an Allied Assassin detachment.

Imperial and Chaos Knights

Imperial Knights however have a problem; they have a rule similar to the Assassins one but it’s missing the whole second part in comparison.

If this Detachment is a Super-heavy Auxiliary Detachment that contains one FREEBLADE unit, until the end of the battle, that unit gains the AGENT OF THE IMPERIUM keyword. Only one FREEBLADE unit in your army can have this keyword.

it gives them the AGENT OF THE IMPERIUM keyword but doesn’t say what that does for them. Which would mean that including an Allied Imperial Knight in a Space Wolves army would cause them to lose Savage Fury as it has no exception to the rule.

If every unit in your army (excluding UNALIGNED units) has the SPACE WOLVES keyword, then every unit that has the Combat Doctrines ability and is in a SPACE WOLVES Detachment gains the Savage Fury ability, below.

Compare this to Chaos Knights and it’s even more bemusing, as their version of this rule does have the exception.

If this Detachment is a Super-heavy Auxiliary Detachment that contains one DREADBLADE unit, until the end of the battle, that unit gains the AGENT OF CHAOS keyword. Only one DREADBLADE unit from your army can have this keyword.
The inclusion of a DREADBLADE AGENT OF CHAOS unit from your army does not prevent CHAOS units in your army from using any rules that require every model from your army to have the same keyword (e.g. Contagions of Nurgle, Cabbalistic Rituals, etc.).

So you can take a Chaos Knight as an Allied Super Heavy Aux detachment in a Death Guard army and they don’t lose access to Contagions of Nurgle. Perhaps this difference will be FAQed, but it’s rules as written for the time being.

Other Examples

Frustratingly for everyone’s favourite robot skeletons the Necrons have rules as written issues, whilst all of their detachment abilities do have a DYNASTIC AGENTS exception, you can’t actually include them in a mixed dynasty detachment because a similar issue to the Tau codex, there’s no exception for them in the last line of their <DYNASTY> rule.

If your army is Battle-forged, you cannot include units from two different dynasties in the same Detachment.

Orks on the other hand have a very similar line in their <CLAN> keyword section but it has an extra clause giving them a exception for certain units.

If your army is Battle-forged, you cannot include units from two different clans in the same Detachment, with the exception of certain units (see Specialist Ladz).

Specialist Ladz includes Ghazghkull, Kaptin Badrukk, Makari, Mad Dok Grotsnik, Zodgrod Wortsnagga, and Specialist Mobs.

Pre-FAQ Suggestion: It’s like that events will rule that all of these abilities/keywords are intended to work the same way especially for SUPREME COMMANDERS, and allow Shadowsun in Tau Armies that are not Tau Sept, and The Silent King in Necron armies that are not Szarekhan Dynasty.
Abaddon would if ruled the same way as Orks are be allowed to lead any <LEGION> force without making the list invalid as he currently would. However ruling this way still prevent Abaddon from leading a CHAOS army (due to detachment limitations), or a none <LEGION> HERETIC ASTARTES one such as Death Guard or Thousand Sons.
Check with your TO or opponent. 

The concludes this quick run through the Arks of Omen Detachment, I’ll be keeping this article up to date with more examples and questions as the season continues. If you have a questions or suggestion for me to cover please drop us a note in the comments below, ask a question in our Ruleshammer form, or head over to r/ruleshammer to discuss.