Exterminatus
Exterminatus is the sheer ass-raping an Inquisitor can unleash upon a planet in Warhammer 40k when he deems the planet past saving. The way said Inquisitor goes about this ass-raping is by killing all life on the planet, if not blowing it the fuck up.
Before you go onto some sort of moral tirade about the morality of blowing the fuck out of a planet, understand the motherfucking context. A world deemed worthy of Exterminatus is one considered past the point where anything can be salvaged from it - whether because it's about to be lost to ravening monsters that will eat fucking everything or because it will be turned into a fucking daemon-infested shit-pit where neither sanity nor time has any meaning. The alternative is fucking glassing a planet and trying to deny it to the enemy or ensure SOMETHING can be fucking saved. It's the last-ditch measure and it's there because the alternative sucks even fucking worse.
Deal with it. Bitching any further will rile the Commissariat. You have been warned.
Ways of Exterminatus
The Inquisition has several means for dealing with infestations:
Just Shoot the Shit Out of It
NOVA CANNON CHARGED
Virus Bombs
Loaded with a biological payload that causes living tissue (plant or animal) to rot and decompose, Virus Bombs are only rarely used now because every time they're used, they feed the fucking Plaguefather. The general gist is that they spread a virus that spreads by contact that causes necrosis of tissues and rapid decay of plant and animal tissues. This immediate rot causes a buildup of flammable gases, which in turn, ignite suddenly, sweeping the area in firestorms. A relentless bombing of these fucking things is what reduced Tallarn from a verdant forest world to the desert hellhole it is now.
Cyclonic Torpedoes
Modified Melta Torpedoes, these capship-fired warheads each generate a series of massive self-sustaining nuclear reactions, which, when fired in bulk, trigger a much larger reaction that causes the devastation to spread and multiply, eventually glassing the entire world with nuclear fire given a sufficient barrage. If you fire enough in the same spot it will break through the crust of a planet causing part of the mantel to erupt out, royally butt fucking the planet in the process(See Fire Warrior End Cinematic). Krieg is an example of a World that survived multiple cyclonic torpedo strikes, though in this case it was on a much smaller scale.
Two-Stage Cyclonic Torpedoes
Same thing but they use the melta charge to burrow into the planet's crust and down to the core. A second charge then goes off causing the planet to break apart Death Star style.
TL;DR
You fuckers just backed Chaos and now you have a daemon infestation? Your planet 'gon git raaaaaaaaaaaaaaped.
