Fusion coil

A fusion coil, also known as a fusion core, is a container used for safely storing and transporting volatile—and often explosive—materials. Different variants of the fusion coil have been manufactured by the UNSC, the Banished, and even the Forerunners. While not classified as fusion coils, the Covenant made use of a similar technology with their plasma batteries.

Variants
All known variants of fusion coil are in the shape of a rectangular prism. They each consist of a metallic outer casing to protect the volatile materials within. The contained materials can usually be seen glowing from within the fusion coils, and the glow becomes more intense as the coil becomes more damaged.

UNSC fusion coil
This variant of fusion coil was developed by the United Nations Space Command to contain superheated plasma. The container is made primarily out of glass and aluminum, and has been jokingly described as the "safest, most intelligent way to transport and store highly explosive compressed plasma energy."

Blast coil
The blast coil is a variant of fusion coil developed and used by the Banished. It contains a highly explosive material that emits a yellow glow. When detonated, the coil delivers a powerful localized blast.

Plasma coil
The plasma coil is a variant of fusion coil developed and used by the Banished to hold plasma, in a similar fashion to Covenant plasma batteries. When detonated, the plasma coil leaves a pool of burning plasma behind.

Shock coil
The shock coil is a variant of fusion coil developed and used by the Banished. It contains electricity and emits a strobing blue light. When detonated, the shock coil emits large amounts of electricity that can arc toward nearby objects. It can also have the effect of an electromagnetic pulse on vehicles and other electronic devices.

Hardlight coil
The hardlight coil is a variant of fusion coil developed by the Forerunners. It contains purple hardlight energy that swirls violently within its casing. When detonated, the hardlight escapes, bouncing off of solid objects and disintegrating any organic material it comes into contact with, much like the hardlight projectiles fired by the Heatwave.

Appearances
The Halo 2-era fusion coils are explosive objects matte brown and rectangular, overlaid with what appears to be austere gold circuitry on the surface. In Halo 3, the fusion coil has been graphically redesigned. The actual plasma coil appears to be an amber, darkly-glowing, cylindrical object within a metallic-gray colored frame. When slightly damaged, the internal coil glows a warning red; when heavily damaged, it glows bright white. Stickers on the frame warn of radioactivity and high voltage. It emits a loud humming noise constantly.

The appearance of the fusion coil in Halo: Reach changed very little from the design in Halo 3. The once yellow glow of the fusion coil is now a dark green. This appearance remains unchanged in Halo 4, with the exception that the internal glow is now blue instead of green. Additionally, Halo 4 contains another form of fusion coil that seems to be filled with a liquid substance that will make a splash when destroyed, and will disintegrate nearby targets. This fusion coil exists in the Majestic, Castle, and Bullseye map packs and in Forge Island.

Fusion coils receive an overhaul in Halo Infinite. They now come in four different varieties, each corresponding with the damage types in the game. There are larger versions of each fusion coil variant as well.

Gameplay
Several shots from any weapon can detonate a fusion coil. In multiplayer, players often may attempt to lay an ambush for a hostile near a coil by tossing a fragmentation grenade towards the coil when the enemy forays near. Alternatively, they can be pushed from above onto passing enemies, exploding on impact with the ground, or used as anti-vehicle landmines at vehicle choke points. They have also been used for explosive jumping and in Forge to create perpetual explosions.

In Halo Infinite, the smaller variation of fusion coils can be picked up and thrown by the player, as well as by enemy Jiralhanae. Like other items that can be held by the player, fusion coils can be picked up from a distance using the Grappleshot. In multiplayer, hitting an enemy Spartan directly with the fusion coil will kill them instantly. Killing an opponent in multiplayer with a thrown fusion coil will earn the player the Kong medal. Larger fusion coils are stationary, but have much more damaging effects when detonated.

Each variant of fusion coil in Halo Infinite performs differently:
 * Blast coils deal kinetic damage in a large area. Large blast coils have an even larger area of effect.
 * Plasma coils are effective against shields and leave a damaging field on the ground, similar to a Veporokk Workshop Ravager's charged shot. Large plasma coils spew plasma in an area before exploding and releasing their fragments, that leave the aforementioned damaging field.
 * Shock coils deal shock damage and have an EMP effect on vehicles. Large coils will spew electricity for a long time when damaged until they explode.
 * Hardlight coils deal hardlight damage and release bouncing projectiles on explosion, like those fired by the Heatwave. Large coils burst in even more projectiles.

Trivia

 * Fusion coils feature prominently in Forge, where players can place a large amount of coils on any location in a map and detonate them. A Bungie Weekly Update described an incident where a beta tester used them, along with the Instant Respawn trick, to turn an Elephant into a "satanic piñata" on the Halo 3 map Sandtrap.
 * When saving a map variant in Halo's Forge, a brief "Terms of Use" agreement is shown. In the agreement, Bungie jokingly references the temptation to exploit these items, adding: "...And try not to build all your structures out of Fusion Coils."
 * In the Halo 3 Manual, under "The Forge - Co-op" It references "Cooperate with your friends to make the work go faster, But don't forget to throw a Fusion Coil at them from time to time."
 * In Halo 2: Anniversary's Forge, there is an EMP variant of the fusion coil.

List of appearances

 * Halo 2
 * Halo 3
 * Halo 3: ODST
 * Halo: Reach
 * Halo 4
 * Halo 2: Anniversary
 * Halo 5: Guardians
 * Halo Infinite
 * Halo: The Television Series