Electromagnetic waves of short wavelength, X-rays are a very intense form of radiation, even more intense than ultraviolet light. Capable of penetrating some thickness of matter. Produced among other things by neutron stars and black holes. Only gamma rays are more intense.
X-rays are ideal for space combat, due to their extremely short wavelengths that allow them to be focused at long distances. However, air is opaque to x-rays, so lasers of these wavelengths are not used in atmospheres or for bombardment of planets with atmospheres. X-rays are tricky to handle, since they cannot be directly reflected and are absorbed by any lens material. Soft x-rays can be focused using complex grazing incidence reflection mirrors, but even these fail for hard x-rays.
Many x-ray lasers rely on a process known as self amplified spontaneous emission, which requires very long lasing chambers and results in unstable beams with very small diameters. Alternately a seed-beam from a shaped crystalline diffractive cavity generator can be ramped up by an x-ray laser amplifier to destructive levels, allowing powerful beams to be emitted which often are used without secondary focusing. The high powered death beam need never touch matter until it reaches its target, thus eliminating the difficult issues of dealing with extreme power levels impinging on delicate focusing optics.
Although x-rays do not have the highest frequency, it's still a bad idea to bask in them. Don't do it! Usually, you should have a lead shield. The thicker the lead is, the more protection. However, there will always be a chance some X-rays get through. Thicker lead just lowers this chance.
Hard and soft x-rays
Low energy x-rays can often range from about 0.1 keV to 10 keV. The range of hard x-rays can be from 10 keV to nearly 1000 keV. The dividing line between hard and soft x-rays is not well defined and can depend on the context.