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.
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.
However, as cool as the Nova Cannon is describe above, one of the problems that is 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? To be fair, the Nova Cannon might be described as the "Sniper-Bullet" of a Imperial Ship then that of...lets say...a Macrocannon which is the "SMG" equivalent. Meaning that the Nova Cannon might have the force and impact of a 100 terraton bomb without the cataclysmic kinetic blast radius, although that still proves a problem since even without the actual bang, its still has enough the power of fucking over a Planet's ecosystem...which still begs the fucking question on what's the fucking point of the existence of this shit when the Imperium already has a weapon that could do similar, if not, even a better job without the hassle of maintenance, time, convenience, cost and authority....the only answer would either be Imperium stupidity or GW not knowing a fucking clue on how Badassary and Grimdarkness needs to be balanced with Logic and Common Sense, or else it would make the entire IoM look being pants on head retarded.