Kill-ships are unique weapon relics used by the Deathwatch. They are actually automated drone-ships guided by the most sophisticated of War-spirits, designed with the singular task of conducting Exterminatus operations in the most extreme of circumstances on worlds that are believed to be completely lost to the Imperium.
To aid them on their mission, Kill-ships are equipped with unique examples of long lost cloaking technology that date back to the Dark Age of Technology. This allows the drone-ship to enter into the target system completely undetected where it can manage to slip past any defensive sentries and reach the target world itself. Once there, the Kill-ship drops its lethal payload onto the planet below whereupon it performs a slingshot maneuver to escape the region by using the world's own gravity well. As the vessel escapes, its actions are felt by the apocalyptic devastation it leaves in its wake that serves as a final act of denial and vengeance upon the enemies of Man.