Energy swords, and more broadly, energy blades, are a broad category of melee weapons commonly used around the Imperium of Sidhae. Similar technologies also exist outside the Imperium.
The cover-all term used in popular vernacular is technically inaccurate, since major technological distinctions exist between true energy blades and energized blades. Nonetheless, all of these weapons are referred to under the same blanket term in non-technical discourse, even military manuals often failing to observe the distinction beyond a passing mention, with the idea that the particular type of blade involved should be (and usually is) obvious from appearance alone.
There are several varieties of energy blades, each having distinct and considerably different operating principle. What all of them have in common is a cutting edge augmented by some form of energy discharge, leading to unmatched cutting power.
Although technically feasible to manufacture in sizes no larger than a medium-sized combat knife, the practical aspects of energizing, cooling and safely operating such weapons dictates that they are usually much larger, typically the size and shape of a sword. Different types of energy blades are carried by various Imperial armed services.
Although sword has traditionally been the mark of a ranking officer, energy swords in particular aren't exclusive to Sidh officers, and are in fact occasionally seen among the rank and file troops. Being non-regulation equipment, they are not generally issued by the armed forces, and are instead purchased by troops at their own expense. The significant costs involved, however, prevent most from buying one and limit them to the standard-issue energized bayonets. Those soldiers who do own an energy sword have usually been awarded one for exceptional valor and distinguished service.
Energy and energized melee weapons do not always take the form of a sword, or indeed a bladed weapon at all.
As explained before, true energy blades are technically distinct from and far less common than energized blades. "Energy blade" is a cover-all term describing several distinct types of bladed weapon, used mainly to distinguish them from non-energized blades.
The most common type of energy sword is technically properly called energized sword, a rather ordinary-looking sword with a hi-tech composite blade and a plasma sheath generator built in the hilt. When activated, powerful currents race down the blade, energizing it and projecting a brilliant sheath of plasma micrometers away from the monomolecular cutting edge. This greatly enhances the weapon's cutting power, especially against armored targets, though proper powered armor can still resist it well - the purpose of energization is to better cut or thrust through weak spots in armor rather than penetrate it out of hand.
A step up in the technology are energized nanite blades, sometimes referred to as EN-blades (and often confused with the similar non-energized nanite or N-blades). Unlike the solid-bladed energized swords, nanite swords are flash-forged from a nanite ferrofluid stored inside the weapon's hilt and formed into a pre-programmed rigid shape by applying precisely-modulated electric current. Like energized swords, these flash-forged blades are then energized by means of a plasma sheath generator. Nanite blades are more compact than traditional swords, but also much more complex to manufacture and command an according price tag, only being seen in hands of highly distinguished warriors.
A further step up in energized blade technology are polymorph weapons, exceedingly rare and expensive nanite-based weapons which usually also include the ability to energize. Unlike conventional nanite blades, which can only assume a single pre-programmed shape, polymorph weapons can be reshaped by the wielder into any number of shapes at will, not necessarily even into bladed weapons. The nanite response time is such that a skilled wielder can reshape the weapon in mid-blow, surprising the enemy who would prepare to defend against a different type of weapon, and constantly switch between different weapon types and accompanying combat techniques in mid-battle. Due to the exorbitant expense of constructing such elaborate weapons, only the greatest Imperial heroes are awarded the honour of carrying one into battle, and only a few dozen are known to exist, all being marvels of technology and engineering each in their own right. The best known energized polymorph weapon in the Imperium is the Manreaper, the Emperor's personal weapon. The Manreaper is traditionally depicted as a longsword, which was the Emperor's preferred form, though its current owner the Empress is known to wield it in the form of a scythe.
Proper energy blades without a solid core, also known as holoblades for their holographic appearance, are a relatively recent innovation not widely seen outside the Imperial Guard and the Order of Judicators. The exact mechanism behind their creation is highly-classified and only vaguely understood by scientists outside these organizations, but the effect of glowing blades materializing literally from thin air (or indeed in the absence of any air) is no less intimidating or deadly for that. Holoblades can be generated using specialized add-ons to Judicator/Guard-issue tacticoms, and are believed to exploit the well-established flash-forge feature also found in less advanced tacticom models, so it is speculated that the mechanism behind their generation actually involves an advanced form of "utility fog", a gaseous cloud of nanites flash-forged into shape by precisely modulated energy fields similar to those used in personal energy shields. Whatever the truth, the enigmatic "hard light" that forms the holoblades is known to conduct electricity (though it is evidently a deliberately triggered feature rather than an innate aspect of the material), the blades being capable of delivering potent electric shocks on contact. At least Judicatorial-issue holoblades are also known to have a non-lethal setting, during which they change colour from orange to green, though it is again unknown if the change of colour reflects an innate change in the blade structure itself, or is simply a cosmetic feature implemented for safety purposes.
All energized blades are powered externally, from the power source of the user's powered armor, and are hence unusable without one, powering down almost instantly if released from grip and thus losing connection. This also serves as a safety feature, preventing the wielder from severely injuring oneself by accidentally touching an energized blade, the pain from such contact prompting instinctive release of the weapon, immediately powering it down. Holoblades are an exception, their power source being compact enough to fit on a tacticom as an addon, and instead use a position-sensing technology to switch to non-lethal whenever dangerous proximity to the user's body is detected.
Judicatrix Serena Romana and rogue Judicator Alain are known to be particularly proficient wielders of dual holoblades. Other Judicators are implied to have a holoblade as a standard tacticom addon, and most probably have attained at least some proficiency in it's use.
Judicator Halko preferentially uses a pair of energized kukris for, in his own words, the more visceral feel of metal slicing through flesh and bone.
Archistrategos Arcadius Drax carries a polymorph weapon named Tyrwing as one of his regalia. Though capable of assuming many shapes, and never explicitly described, his preferred form is suggested to be a thunder hammer.