Deathwatch Mission Tactics
At the start of your Command phase you can select one of three Mission Tactics; it is active until the start of your next Command phase and applies to all DEATHWATCH units from your army, and each one can only be selected ONCE PER BATTLE. Furor Tactics gives Deathwatch weapons [SUSTAINED HITS 1]; Malleus Tactics gives them [LETHAL HITS]; Purgatus Tactics gives Deathwatch attacks [PRECISION] on a Critical Wound. The once-per-battle clause is the whole design: you get three good turns out of five and you have to decide which turn each one belongs to. Note the keyword gate carefully, because it catches people out: the rule reaches DEATHWATCH units only, not every Ordo Xenos model, so an Ordo Xenos Inquisitor gets nothing from it.