Tachyon

Tachyons are particles that travel faster than the speed of light. Tachyons do not behave in the same way that non-tachyonic particles do with respect to general relativity. Unlike normal matter, which requires more and more energy to be added to accelerate close to light speed, tachyonic particles require large amounts of energy decelerate them to the speed of light, and with less and less energy they are ever faster.

The ultimate low energy state of a tachyon is constant bearing, where a tachyon is travelling at infinite speed and thus occupies a constant trajectory around the universe, theoretically being at all places along that trajectory at all times. As constant bearings, tachyons are in fact faster than even spacetime itself, as spacetime itself has a finite speed far in excess of the speed of light.

Any mass/energy added to the tachyon causes it to slow down to a finite speed (although an infinite amount of mass would be required to decelerate a tachyon to sub-light speed). The phenomenon of mass decelerating a tachyon is known as spacetime drag or higgs drag.

Uses
Since tachyons travelled faster than the speed of light, they can be used in communication. Constant bearing tachyons can theoretically be used to offer instantaneous communication. In practice, however, any interference with the constant bearings gives them mass and therefore finite speed. Tachyon communication is, however, useful in scenarios where quantum communication has been ruled out and light speed communication is too slow or otherwise also ruled out.

Tachyons can be weaponised, but ordinarily they pass through normal matter without interacting. Therefore, tachyon weapons must be developed so that they interact with normal matter in order to release energy and cause damage.