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Question on the Brood Brother's rule. The rule seems to give a specific exemption for matched play that allows you to take AM units alongside GSC units (but not all Nid units) even though they do not share a faction keyword. This specifies, though, that you must have one GSC detachment for each AM detachment.

However, the final sentence is throwing us off. It states that, "In such cases, simply ignore Astra Militarum units when choosing your army's faction." This seems to allow a matched play Tyranids army using 1,001+ points (and thus allowing 3+ detachments) to declare their overriding faction as "Tyranids" as their army keyword to have the following detachment make-up:
- Detachment 1: Tyranids faction - Could take any Nids or GSC units with the Tyranids faction keyword.
- Detachment 2: Genestealer Cults faction - Can only take GSC units, not normal Nids.
- Detachment 3: Astra Militarum faction - Can only take AM units.

Even though reading Brood Brothers RAI seems to imply that you can only mix GSC/AM and only in separate detachments. However, the concluding sentence gives a much broader exemption than explained earlier in the rule because the AM units are simply ignored for faction purposes so long as you meet the detachment faction requirements specified in the rule. Through that, you can get some standard Tyranid detachments mixed in.

Thoughts? Does the rule allow Nid/GSC/AM mixed armies or is it only GSC/AM armies? Why?

Thanks!

Yes, it allows mixed armies because you ignore the AM units when deciding what your faction keyword is. If you pick Tyranids, it lets you take GSC and Vanilla Nids by virtue of the Faction keyword TYRANIDS, and lets you take AM units because you ignore the AM keyword due to Brood Brothers.

Perfectly fluffy to represent infected guard turning on the other humans when nids make planetfall.

Negative. When you build an army before making any detachment you choose a Faction. All of your detachments must then be from this faction.
To take AM you get 1 detachment for every GC detachment you take. Tyranids do not have the Genestealer Cults keyword and thus cannot be taken in a Genestealer Cult army.

So it's either a Tyranid faction army and you can mix tyranids, and GC's however you want in your detachments OR it's a Genestealer Cult Faction army in which case you can only take units in your detachment with the Genestealer Cult keyword, and then you can take 1 AM detachment for each Genestealer Cult detachment.

Wagguy80 wrote:
Negative. When you build an army before making any detachment you choose a Faction. All of your detachments must then be from this faction.
To take AM you get 1 detachment for every GC detachment you take. Tyranids do not have the Genestealer Cults keyword and thus cannot be taken in a Genestealer Cult army.

So it's either a Tyranid faction army and you can mix tyranids, and GC's however you want in your detachments OR it's a Genestealer Cult Faction army in which case you can only take units in your detachment with the Genestealer Cult keyword, and then you can take 1 AM detachment for each Genestealer Cult detachment.

Hey guess what, not only are you totally wrong from a rules standpoint, the official FAQ's answer this. https://www.warhammer-community.com/2017/07/02/warhammer-40000-faq-now-available-july2gw-homepage-post-1/

Index Xenos 2 FAQ wrote:Q. Can I have a Battle-forged army that consists of one Detachment of Genestealer Cults, one Detachment of Astra Militarum (using the Brood Brothers rule) and one Detachment of Tyranids?
A. Yes.

You don't "choose" a faction when you're making your army, you choose a common faction keyword, which GSC allow you to ignore for AM units.