[Reference] Brood Brothers

When mustering a Battle-forged army, for each GENESTEALER CULTS Detachment you include in your army, you can also include one ASTRA MILITARUM Detachment, even though the units in such a Detachment do not have any Faction keywords in common (this does not prevent your army from being Battle-forged). These ASTRA MILITARUM Detachments are also known as BROOD BROTHER Detachments and have the following rules:
• You can only include units with the <REGIMENT> or UNALIGNED keyword in a BROOD BROTHERS Detachment.
• When you include an ASTRA MILITARUM unit in a BROOD BROTHERS Detachment, replace the IMPERIUM and <REGIMENT> keywords in every instance on its datasheet with the BROOD BROTHERS keyword.
• BROOD BROTHERS models cannot be selected as your WARLORD or have a Warlord Trait.
• BROOD BROTEHRS models cannot be given any Relics.

The inclusion of BROOD BROTHERS units in your army does not prevent GENESTEALER CULTS units in your army from using the Crossfire ability (pg 89), provided that the total Power Rating (or points values in a game that uses them) of BROOD BROTHERS units from your army is no more than 25% of your army's total.

BROOD BROTHERS units in a BROOD BROTHERS Detachment gain the following Regimental Doctrine (see Codex: Astra Militarum):
Loyalty to the Cult
• Add 2 to the Leadership characteristic of models in a unit with this Regimental Doctrine.
• INFANTRY units with this Regimental Doctrine gain the Unquestioning Loyalty ability.

Each time a saving throw made for a <CULT> CHARACTER model from your army is failed, you can select one other friendly <CULT> or BROOD BROTHERS model with this ability within 3" of that CHARACTER model and take an Unquestioning Loyalty test. To do so, roll one D6, adding 1 to the result if that CHARACTER model is a PATRIARCH model: on a 4+, the test is passed and that CHARACTER model does not suffer damage. Instead, the friendly model you selected is destroyed and the attack sequence ends.