Exterminatus

"Some may question your right to destroy ten billion people. Those who understand know that you have no right to let them live."

- Exterminatus Extremis

Exterminatus is the name of a terrible order given by the authorities of the Imperium of Man to destroy an entire planetary biosphere and all life upon a world in extreme circumstances where a potential or actual threat to the entire Imperium exists on the planet and might spread to other Imperial worlds. Exterminatus is typically only used as a last resort, when the price of holding or retaking the world for the Emperor is deemed too high, or the threat of mutation, disease or Chaotic heresy spreading to the rest of the Imperium is too great to contain by conventional means. Thus Exterminatus has very rarely been used on vital or valuable planets to the Imperium. Typically, the order may only be given by a high-ranking Imperial Commander in the Imperial Guard, Imperial Navy or the Space Marines, or by a full member of the Imperial Inquisition. Exterminatus can be carried out in many different ways, as outlined below.

Methods
Note that with the exception of those methods listed under the section Other, the rest of the weapons described here are deployed from warships in orbital space.

Orbital Bombardment
An orbital bombardment from Imperial warships is the simplest means of carrying out an Exterminatus, as it can be done solely through the conventional firepower already available to the Imperial Navy. The power of an orbital strike can range from small precision strikes, not unlike that of modern surface-to-surface missiles, to the unleashing of full thermonuclear blasts. Typically, specialised munitions are used, designed to destroy all life on a planet's surface and potentially any entrenched even in the planet's crust.

Virus Bombs
Imperial Virus Bombs release a special virus known as the Life-eater Virus that was genetically designed to quickly spread and destroy all organic cellular structures it infects, reducing all planetary life, whether flora or fauna, to an undifferentiated organic sludge of biochemicals. This process of cellular decay also produces huge volumes of flammable gases as a by-product, which is later ignited and turns the atmosphere into a firestorm that usually takes out whatever life may be left. Only those who may have escaped into especially-deep underground shelters can survive a Virus Bomb attack. The population of Tallarn managed to escape complete destruction this way, but their once-verdant world was forever transformed into a desert. Virus bombs are known for their extreme speed; in the fiction surrounding the Horus Heresy, they completely wiped out Istvaan III's entire population of 16 billion people in only minutes.

Cyclonic Torpedoes
Cyclonic Torpedoes are much more immediate in action than virus bombs, having enough power to ignite a planet's atmosphere following their detonation. The blast radius is large enough to be seen from space and the detonation powerful enough to crack a planet's crust and destabilise its core. Only the forces of the Inquisition and the Adeptus Astartes are known to have access to Cyclonic Torpedoes and can use them to order an Exterminatus.

Two-Stage Cyclonic Torpedoes
Two-Stage Cyclonic Torpedoes, which are a more exotic form of the standard Cyclonic Torpedoes, are the most common of a special class of rare Exterminatus weapons, designed for use against atmosphere-less or biologically-void worlds (Necron Tomb Worlds being the main example). These torpedoes possess two-stage warheads: The first stage is an unusually powerful Melta Charge that bores straight through a planet's surface all the way down to its core. The second stage is a modified Cyclonic Charge that destabilizes it, in most cases physically destroying the planet from the inside out.

Other
During the 13th Black Crusade, Lord Castellan Ursarkar E. Creed, the Planetary Governor of Cadia and Imperial Commander of the entire Cadian Sector of the Segmentum Obscurus, ordered a massive nuclear fusion Plasma Generator (Generatorium), on the prison world of St. Josmane's Hope to be overloaded when it seemed that Chaos would finally take the world. Rather than a colossal meltdown, the reactor exploded with furious energy and blew the entire planet apart with the force of its blast.