Fusion coil
From Halopedia, the Halo wiki
The Fusion core is a United Nations Space Command device used as an energy source in ground operations.
Aesthetics
They are explosive objects matte black in color and rectangular in shape overlaid with what appears to be austere gold circuitry on the surface. They can be found on various Halo 2 multiplayer maps, such as Lockout and Ivory Tower.
Pyrotechnics
Several shots can detonate these voliatile core, often exploding with suffecient force to severely wound or kill a nearby Sangheili or SPARTAN-II. The explosion is a bloom of pink superheated gas and sparks. In multiplayer, players often may attempt to lay an ambush for a hostile near a core by rolling a Fragmentation Grenade towards the core when the enemy forays near. Alternatively, they can be pushed from above onto passing enemies, exploding on impact with the ground (sometimes a shot is needed to blow it up if from low heights) or used as an antivehicular land mine (sometimes with more than one core) at vehicle choke points.
Halo 3
They will return again in Halo 3 and be just as annoying/lethal as ever, but this time, they feature "Safety Features", stickers warning of possible hazards. Not surprising since the entire core IS A HAZARD! They can be used for defense and offense still, and are apparently still mobile and able to be pushed around with ease. They probably appear in many Human levels in Halo 3 Multiplayer, though this is not confirmed. They are also said to respawn after an amount of time after destruction.
Also, as stated in a Bungie.net weekly update, the fusion cores will have a new feature claimed to revolutionize their practicality.
The Fusion Core may be related to the also UNSC Deuterium Fusion Core, although this hasn't been confirmed.
