Armageddon Gun
The Armageddon Gun is the main weapon of the Planet Killer and is an ECKS BAWKS HUEG cannon that is the single largest gun ever mounted on a moving platform. This is basically the Deathstar of WH40k. Seriously, look at the damn thing.
Overview
The Armageddon Gun consists of not one, not two, but seven giant-ass barrels of doom, each as long as the Planet Killer itself. If anything, the Planet Killer's total length is 80% gun. This seven prongs of death are so big, that only the Imperium's Annihilator Cannon could rival it.
When the Planet Killer wants something dead, it simply points its barrels, waits for it to charge up, and anything within its killzone gets promptly vapourised. True to its name, the Armageddon Gun can easily destroy planets, albeit it may take several seconds before it burrows deep in the core and destabilizes the damned thing.
Now, what the Armageddon Gun is, we are not too sure. It can be a ginormous laser or it can be a supped-up melta. All we do know is that it is most definitely a form of directed-energy weapon that can punch clean through a planet's crust and right down to the core.
Battlefleet Gothic: Armada II
In Battlefleet Gothic: Armada II, the Armageddon Gun is the single most powerful weapon in the game. Full stop. This is a one-hit kill weapon. Nothing survives, not even fellow Titan ships. You just point, aim, and say "Bye-Bye" to anything still dumb enough to stay within its several-second charge phase. Use this as a form of suppressive fire to kite enemy ships into a disadvantageous spot.