Primus

Friendly units with the Genestealer Cults Faction within 12" of this model have the Hatred special rule.

Units with this special rule that Infiltrate, or that arrive from Reserve or Ongoing Reserve, can chose to roll on the Cult Ambush table instead of deploying or arriving from Reserves normally. Unless otherwise specified, ambushing units move onto the table as described for other Reserves and cannot move any further during the movement phase. Units cannot use the cult ambush special rule whilst they are embarked inside a vehicle.

A model with this special rule automatically passes Look Out, Sir attempts, and can even make Look Out, Sir attempts when fighting in a challenge.

Instead of moving in the movement phase, any unit with this special rule that is not within 6" of any enemy models, can be removed from the battlefield and placed into Ongoing Reserve. A unit cannot Return to the Shadows in the same turn it arrives from Reserves or Ongoing Reserves, and cannot do so whilst embarked in a vehicle.

Units that contain at least one model with this special rule are deployed last, after all other units (friend and foe) have been deployed. If both sides have Infiltrators, the players roll-off and the winner decides who goes first, then alternate deploying these units.

Infiltrators can be set up anywhere on the table that is more than 12" from any enemy unit, as long as no deployed enemy unit can draw line of sight to them. This includes in a building, as long as the building is more than 12" from any enemy unit. Alternatively, they can be set up anywhere on the table more than 18" from any enemy unit, even in plain sight.

If a unit with Infiltrate deploys inside a Dedicated Transport, they may Infiltrate along with their Transport. 

A unit that deploys using these rules cannot charge in their first turn. Having Infiltrate also confers the Outflank special rule to units of Infiltrators that are kept as Reserves.

If a unit has both the Infiltrate and Scout special rule, that unit can deploy as per the Infiltrate special rule and then redeploy as per the Scout special rule.

Independent characters can join other units.  They cannot, however, join units that contain vehicles or Monstrous Creatures.  They can join other Independent Characters, though, to form a powerful multi-character unit!  Independent Characters pass Look Out, Sir rolls on a 2+. A unit that contains one or more Independent Characters does not need a double 1 to Regroup if reduced to below 25% of its starting numbres, but instead tests as if it had at least 25% remaining.

Full details in BRB. Rule too long to copy verbatim.

This rule is often presented as Hatred (X) where X identifies a specific type of foe. If the special rule does not specify a type of foe, then the unit has Hatred against everyone. This can refer to a Faction, or a specific unit. For example, Hatred (Orks) means any model with the Ork Faction, whilst Hatred (Big Meks) means only Big Meks. A model striking a hated foe in close combat re-rolls all failed To Hit rolls during the first round of each close combat.