These detachments specialise in fighting psykers and daemons.
Though highly secretive, the elite Grey Knights of the Imperium are a prime example of such forces.
This niche ability is only for use by agreement between the players, for special scenarios. It does not have a points cost — you need to allow for its likely value in the overall context of the scenario and the composition of the armies that the players will use.
Where your detachment has the Daemonbane ability:
In close combat, double the Assault value of each unit in the detachment that is in base contact with an enemy Daemon or Psyker unit.
When you shoot with this detachment, apply a -1 modifier to the Armour value of each Daemon unit and each Psyker unit in the target detachment.
We define a Daemon unit as any of these units in the Chaos faction: any unit in a Daemon Detachment; any Daemon Prince unit.