A spacecraft-mounted weapon I put together for the Lejendairidan fleet. It probably breaks several laws of physics as we understand them but here it goes:

The idea is the device generates a self sustaining "shell" of superheated plasma and exotic matter. This shell maintains its energy through its own tightly bound magnetic field driven by an "infusion" of ultra-dense metallic hydrogen and metallic helium particles. Once the shell is generated over a matter of seconds or minutes (depending on the intended yield of the shell) it is rapidly accelerated towards a significant fraction of the speed of light using the device's immense assembly of intensely strong electromagnets. A ring of additional electromagnets seen at the end of the firing channel can induce a "twist" upon the velocity of the shell at the last microsecond to steer the projectile towards an intended target.

Planetary Grade Guided Plasma Elements and their related devices are among the only weapons of potential-mass-destruction of Lejendairdan make which can be legally obtained (with sufficient clearance.) The Lejendairidans have a steep opinion regarding the use of such potently devastating weapons especially after what happened 260 standard years ago.

Weapon Specifications

Length: 90 Meters
Width: 10-36 Meters
Power Supply: High Yield Fusion or Anti-Matter Reactor via Vessel Main or Armament Power, 800-4,000 Petawatt pulse

Firing Channel Width: 4.6 Meters
Projectile Speed: 0.1-19% Speed of Light
Projectile Type: Self-Maintaining Plasma Shell, Toroidal and Ellipsoidal formations

Modeled and rendered with Blender, edited with Gimp