Relativistic Bombardment

Relativistic Bombardment is less of a 'weapon' in traditional sense of the word as it is a procedure. Theoretically every high calibre MAW is capable of pulling a relativistic bombardment, all it's users have to do is switch of all limiters and willingly risk the barrel being broken or the gun exploding due to overcharge. You can even pull that with ships if you spent a while accelerating them into high speed (though the sheer size of them makes it possible to do the same ammount of damage with less sped).
Relativictic Bombardment is also used via a specialized Relativistic Cannons, massive spinal mounted weapon systems that can project solid rounds at speed close to lightspeed. A single round has enough kinetic force to split apart continents and vaporize oceans. It is almost too powerful to use, since it doesn't even leave ruins behind - it obliterates everything.
Still, most major countries have at least a single leviathan (in most cases a colossi) armed with a relativistic cannon, just in case something truly terrible happened and the destruction of the world was considered a lesser evil.