Title explains all - can be fantasy, sci-fi, or some gun-type thing that could exist, but doesn’t.
The Alchemist - http://en.wikipedia.org/wiki/The_Neutronium_Alchemist
Best doomsday weapon ever - basically generates ultrastrong gravity. On strong setting, can create a black hole. Drop it into a star - bye-bye star!
On the weak setting, it rips neutrons from atoms, gradually causing a build-up of neutronium on the outer surface. It dropped into something dense (like a gas giant or star) the build-up because significant, and exponential. The result - something not quite a black hole that makes the star go nova - bye-bye solar system!
Coherence gun - http://en.wikipedia.org/wiki/Miracle_of_Science
Kick ass concept. Kick ass name. Not a decoherence gun, those things are lame. The coherence gun collapses the wavefunction of electrons in atoms, (making them coherent,) which sends the electrons crashing into the nucleus. Woot!
This, too, would generate neutronium. Meh, some people like leather.