Extremis Sanction ^^Officio Assassinorum^^ units from your army can use the Overkill, Soulless Horror and Shieldbreaker abilities twice per battle, instead of once per battle (but cannot use such an ability more than once in the same battle round).
When mustering your army, each ^^Officio Assassinorum^^ unit from your army has the relevant Extremis ability, and you must increase the points cost of each of those units by the amount shown. If this causes your army to exceed the points limit for the battle you are playing, you cannot include that unit in your army.