via the Warhammer Community
https://www.warhammer-community.com/2017/09/18/the-armies-of-firestorm-hammerhal-sep18gw-homepage-post-4/
What are Great Cities allegiance abilities?
With the new General’s Handbook out, there are more allegiance abilities than ever to customise your collection. Firestorm allegiance abilities act as an additional allegiance ability for your army on top of any others it may already have; a Hammerhal Darkling Covens army, for instance, would benefit from both the Hammerhal traits and the Darkling Covens traits.
Hammerhal
The armies of Hammerhal are renowned for their bravery in the face of overwhelming odds, represented by Pride of Hammerhal:
This trait makes list building an interesting balancing act. Larger units benefit more from increased Bravery, but the more models you take, the less likely you are to claim the most powerful benefits.
Who benefits?
Best Factions
As a fairly elite army, the Fyreslayers obviously benefit greatly from some bonus bravery. We’d recommend using your Rune of Fiery Determination (a Fyreslayers allegiance ability) in the first couple of turns to ignore Battleshock, then, as the battle goes on and you begin to take casualties, you can switch to the Hammerhal allegiance abilities. We’d recommend taking one full-sized unit of Hearthguard Berzerkers as the core of your army, with the rest of your force made of elite monsters and heroes like the Auric Runefather on Magmadroth.
Wanderers armies with the Hammerhal allegiance ability are going to be incredibly difficult for your opponents to finish off. The low model count of your Wanderers army means that you’re likely to be claiming decent bonuses to your Bravery, with Defiant Hunters providing further insurance against Battleshock with re-rolls to failed tests. If your opponent wants to finish a Wanderers unit, they’re going to have to kill them all off – a task that’s easier said than done with the Realm Wanderers and Melt Away abilities allowing key units like Glade Guard and Sisters of the Watch to outmanoeuvre their foe while peppering them with ranged attacks.
Finally, a standard Order army works very well with Hammerhal forces, allowing you to benefit from an enormous pool of potential units. In this case, the Armies of the Great Cities: Hammerhal boxed set has a great core of units to base your army around; elite units like the Stormcast Eternals Liberators and Demigryph Knights make for a durable front line with a low model count, while your more numerous units like the Freeguild Guard and Freeguild Handgunners will be much more resistant to a bad Battleshock test.
You’ll be able to get started with your own Hammerhal Army this Saturday, when Warhammer Age of Sigmar: Firestorm and the Warriors of the Great Cities boxed sets are available to pre-order.