Type-1 energy sword
From Halopedia, the Halo wiki
The Type-1 Energy Weapon/Sword (T-1 EW/S), commonly known as the energy sword or the plasma sword, is a traditional Sangheili close-quarters weapon. The energy sword is intended to express its owner's personal skill, clerical honor, and combat viability.
The energy sword is the signature weapon of the Sangheili and has been their chief weapon of nobility since its creation during one of the Ages of Discovery. The Sangheili pride themselves on their skills with the weapon, which they regard as holy, and believe that it is better for a Sangheili to fall on his sword to redeem his honor than to die dishonorably. The Sangheili are very strict on who can be trained in the art of swordsmanship; in civilian life only aristocrats are permitted to wield energy swords. Sword wielders are not eligible for marriage, though they may breed with any female they choose, married or otherwise, to ensure successful transmission of "swordsman" genes.
Within the Covenant military, use of the energy sword is not restricted to aristocrats. Sangheili Minors are generally not permitted to wield the sword because of the high regard in which it is held, though exceptions have been witnessed. It is unknown whether this extends to Majors, as at least one has been witnessed using an energy sword. Storm troops, Ultras, Zealots, Generals, special operations, stealth troops, and honor guardsmen commonly use energy swords.
The energy sword's design is seen as the logical evolution of an ancient lineage of traditional Sangheili weapons, most notably the twin-scythe and the curveblade; superficially, the twin-scythe's influence on the energy sword's design is unmistakable. Technologically, the energy sword succeeded an older type of Sangheili sword known as the burnblade, which was used in the early years of the Covenant. Modern energy swords are manufactured by the Merchants of Qikost, a prominent group of arms makers from one of Sanghelios' moons. The energy sword consists of a curved hilt, which houses an energy storage and generation device, as well as the blade projector; that forms a blade of superheated plasma, stabilized by two small magnetic field generators built into the handle of the weapon—this forms and contains the oval shaped, ionized blades for which the weapon is recognized. The weapon is powered by a small battery that supplies power to the plasma generation device and magnetic field generators which generate and contain the plasma needed to form the blade. The battery's energy is reduced for each successful strike. Each strike from the sword will drain the battery by 10% of its maximum energy output. Once the battery power is fully depleted, the sword will deactivate unless recharged. The horizontal hilt forces the wielder to use a punching motion to stab rather than the thrusting motion of most knives and swords.
Mass-produced energy swords feature a roughly W-shaped hilt and have little variation in blade design. Specifically, some models have more rounded blades in lieu of the more common angular style. However, more drastic variations exist. For example, Arbiter Ripa 'Moramee's matching pair of custom-designed swords had basket-style hilt guards and radiated a high degree of ionized plasma, whereas most energy swords contain the blades' plasma with greater efficiency. Some Sangheili special forces units wield blood red-colored energy swords which (ironically, despite their color) burn hotter than more common models. Arbiter Thel 'Vadam's personal sword, the Prophets' Bane, was pale salmon-colored and had a wider blade compared to mass-produced swords.
Some energy swords contain a fail-safe mechanism that can permanently disable the weapon if the Sangheili wielding it drops it. When dropped, the failsafe engages by deactivating the magnetic field before dispersing the plasma. The plasma then consumes the handle and thus destroys the weapon. The fail-safe is included to prevent enemy infantry from acquiring the weapon and using it, and gives insight to Sangheili tactics.
Each energy sword is marked by its creator and first owner, but often their legacy is forgotten. The Pelosus-Pattern Blade was utilized by the original Covenant for the grand majority of their reign, and featured very minute alterations in design and function over the course of its tenure. With the dissolution of the Covenant following their defeat at the hands of the humans, the Pelosus blade would see diminishing use as the now-peaceful Sangheili parted ways with the humans on good terms. Later, with the rise of Jul 'Mdama's chapter of the Covenant, a new Domotos-Pattern Blade was instated as the new Covenant's energy blade. Though its usage was considerably more brief than its predecessor, and its design remained identical throughout it's reign. The Battle for Zeta Halo saw a third iteration, the Meluth'qelos-Pattern Blade developed and brandished by the combined forces of the Banished. Though its still young, it has already seen a fair degree of individual iteration, with two Banished Sangheili developing their own variants for personal use. 
The Type-1 energy sword is an extremely devastating and powerful melee weapon when applied during in close-quarters combat. A single strike can even penetrate the energy shield systems, armor, and flesh of a Spartan or a Sangheili. This is due to the design of the weapon, which utilizes ionized gas rather than traditional shaped, solid matter—because the energy sword lacks any solid material in which to hold, or impale anything, the sword does not cut or carve in the traditional sense, but rather boils anything it comes into contact with. The Type-1 Energy Weapon's edge is therefore extremely volatile, being able to slice through even the toughest metals like a hot knife through butter, including ODST battle armor and the Titanium-A class plating of UNSC destroyers.
Injuries caused by the weapon are often gruesome. Stab wounds by the energy sword are, in most cases, fatal — as the blade passes through the body, the organs and tissues are cauterized by the extreme temperatures produced by the blades. Body fluids in the area of the stab wound are flash-vaporized upon contact; organs within the area of the stab wounds can suffer life-threatening damage dependent upon which organs are struck. The effects of fluids trapped in organs or arteries and the expansion of heat could lead to ruptures or small explosions, causing additional damage to the victim.
Survival is minimal at best and in the case of non-lethal strikes being administered, proper medical treatment must be applied as soon as possible to ensure permanent survival. Because of the weapon's sheer destructive damage, common forms of fatality to victims include total impalement, loss of limbs, bodies being bisected and decapitation.
The Type-1 Energy Weapon's most glaring downside is that it is purely a close-quarters weapon; any combat engagement outside of the effective range puts the wielder at risk. It is for this reason that the Sangheili approach sword combat on the battlefield in two different ways, by funneling the enemy into close-quarters to allow them to strike, and/ or to apply a form of stealth using both technology and technique. Without these, the user is vulnerable to attack by all forms of ranged attack, especially from sniper-based counterattacks. Infantry with Overshields are also more resistant when facing this weapon as it can take two strikes before the shield system fails and then another for the killing blow.
Furthermore, the Type-1 Energy Weapon is vulnerable to other magnetic fields, as they can block, disrupt, or possibly even alter the functionality of the sword. Two energy blades clashing together create a small disruption of energy as a result of the same type of magnetic fields clashing together; this clash has a slight area of effect damage that can affect the combatants.
As with most other Covenant weapons, the Type-1 Energy Weapon is battery powered, requiring the user to either recharge the weapon (which can never be performed in-game) or discard it when the power is depleted. Although the handle can generally be used as a club, the Energy Sword's combat effectiveness at that point is minimal. The strength and lethality of the sword in the games are compensated for the sake of gameplay.
Last but not least, the Type-1 Energy Weapon can sometimes expose the position of a Sangheili warrior employing active camouflage due to the weapon's energy output.
Changes from Halo: Combat Evolved to Halo 2
In Halo: Combat Evolved, the energy sword has a cyan glow, a far less defined edge and makes a dull clunk when striking an object. It is only used by Stealth Sangheili and Zealots. The player cannot wield the energy sword, due to a fail-safe mechanism that vaporizes the weapon when it is not released properly (e.g. being dropped upon death) by the sword-wielder.
In Halo 2, the energy sword is blue with purple undertones, with electrical energy crackling on it when the blade is activated. The white glow seen in Halo: Combat Evolved is also replaced with a more glass-like transparency. In the campaign, it is only used by higher-ranking Sangheili, such as special operations units, Ultras and Zealots, though players can now wield the sword. The sword in campaign has a battery and can only be used effectively against a limited number of enemy forces before the battery runs out. The sword is usable in multiplayer with an infinite battery power. The energy sword has two modes of attacking; a lunge attack and a swing attack.
Changes from Halo 2 to Halo 3
In Halo 3, the sword has more of a light cyan tinge, similar to its appearance in Halo: Combat Evolved, but still has a tint of purple near the handle. It also has an angle near the handle, whereas in the two previous games it was curved. Players are now able to parry with the Sword. The power exerted by the swords hitting each other will drain both players' shields, but will not kill them unless one makes the fatal blow. The sword's lunge attack has been reduced for balancing purposes. Aside from these changes in design and functionality, the power of the sword remains unchanged. In multiplayer, the sword now has a limited battery supply.
Changes from Halo 3 to Halo: Reach
In Halo: Reach, the sword is now darker in color. When activated, it is capable of lighting up the surrounding area. When unused, the sword rests differently on the SPARTAN and Sangheili character models. The energy sword appears to have been balanced out compared with its earlier incarnations, having a slightly faster swing than its Halo 3 incarnation, its lunge attack now does the same damage as a normal swing, the range has been reduced, and can now be blocked by a well-timed normal melee. Players can no longer lunge immediately after a normal swing. The energy sword also has its own assassination animations when used against Elites or Spartans, replacing the Spartan combat knife and the Sangheili energy dagger; these assassinations do not deplete the sword's charge, nor do they count toward the weapon spree medals Sword Spree, Slice 'N Dice, or Cutting Crew. Elites also hold the sword back more than a Spartan when used as an active weapon. When a player is playing as an Elite and uses the Energy Sword as a primary weapon, the camera will lower a bit; this is due to the Elite hunching down. Finally, parrying does slight damage and can kill a player.
Changes from Halo: Reach to Halo 4
Contrary to Halo: Reach, Halo 4 does not feature the possibility to block a sword lunge with a simple melee. Killing an opponent with an energy sword no longer rewards the player with a Pummel medal, it is instead replaced by the Sword Kill medal. The battery is also drained with every hit, including against walls and floors, so it's far less viable against Hunters and other powerful enemies.
Changes from Halo 4 to Halo 2: Anniversary
Like the original Halo 2, players cannot parry with the sword. An alternate version of the sword with a red-colored blade, called the Infected Energy Sword, can be used.
Changes from Halo 2: Anniversary to Halo 5: Guardians
The Halo 5: Guardians sword retains the ability to parry from Halo 4. Unlike in past titles, players can aim with the sword to increase its effective range, and the weapon provides a noticeable speed boost which is enough to make the player visible on the motion sensor while crouching. Three unique versions of the energy sword are available for use: Ravening Sliver, Vorpal Talon, and the mythic Prophets' Bane.
If the energy sword's battery is depleted, the player cannot lunge with the sword.
Changes from Halo 5: Guardians to Halo Infinite
Non-canon and dubious canon appearances
Energy swords are used by Covenant Sangheili against humanity.
Screenshots and artwork
List of appearances