Nova Cannon

The Nova Cannon is a weapon in Battlefleet Gothic, no THE weapon in Battlefleet Gothic, noteworthy for the fact that it creates small blast templates. This may not sound all that impressive until you remember that in Battlefleet Gothic the centre of the base represents a spaceship the size of a hive city. So on this scale, a nova cannons blast is equivelant to the K-T extinction that wiped out the large Dinosaurs. In one shot. Just think about that. That's the sort of Grade A fucking insane "Humanity Fuck Yeah" ridiculously violent scale we're dealing with in Battlefleet Gothic. This is why it is awesome. It allows something like this to exist.

Although the question usually ask is...if each Imperial Ship the size of a Cruiser could be armed with one, especially one with ridiculous firepower even in the context of WH40K...then what's the point of deploying an Exterminatus weapon in the first place?

What this mad bastard actually is varies depending on who you ask. Is it a railgun? A vortex missile? Every damn bit of explosive on a forge world shoved in one container? No-one is entirely sure, but one thing is certain. If an enemy ship has a Nova Cannon you stay the hell away from that magnificent beast unless you wish to suffer sudden massive existence failure. I'm not even joking. Just stay the fuck away.

Alternatively you can just spread your fleet. Seriously. Nova-cannon equipped ships are only effective against clustered formations, and generally despite it's awesome AOE Nova blast is not powerful enough to guarantee one-shotting anything caught in it, unless it's an escort, an interceptor wing or torpedo volley. Additionally this thing have a looong reload time, and it's even longed it the ship crew is not of the top quality. Hence why imperial fleet admirals despise Dominator class cruisers (ones armed with Nova Cannon and little else) and always prefer more reliable ships unless fighting 'Nids and Orks, whose ships DO tend to form a tight formations.