Engineers are shown briefly in Halo 4 in cutscenes and are inferred to still be a part of the (Storm) Covenant, who have sided with the Prometheans.
Engineers are the human term for the Covenant Huragoks. This species floats in the air and are capable of fixing anything from Human, Covenant, or Forerunner technology. The species is used as a slave race to the Covenant forcing them to excavate Forerunner ruins, repair Covenant military technology, and even using extras as improvised suicide bombers.
The greatest secret to the Engineers is the fact they are not a natural biological species. They were created by the Forerunners as sentinet supercomputers. The nanomechanical tissues and organs are indistinguishable from the natural thing even to trained eys unles observed under very powerful scanners. Engineers look like spikey balloons with tentacles coming out the bottom of the creature. These tentacles can split in almost microscopic cilia that allow the Engineer to fix things ant incredible speed. They have been show dismantling and rearrangeing whole vehicles in a matter of seconds and making the new version run atleast as well as the orignal. Engineers can surive in oxygen rich enviroments as well as methane based ones
Powers & Abilities
While no natural technology has been observed from Engineers they themselves may be responisble for atleast the non military technology used by the Covenant. The Engineer "Lighter than Some" built a plow in order to make reprimands for killing a human(see above) and that plaow was confiscated and turned into the Brute Chopper, a one maned attack craft. Engineers in the field are usually armed with a Covenant Overshield for added protection, not only for them but for the Covenat forces since firled Engineers are used as suicide bombers. Considering their ability Engineers could concievably build anything the Humans or Covenant could.
The Enginners were designed by the Forerunners to be organic supercomputers. They can process data and information faster than the Covenants best systems easily. They can repair any form of technology whether it be Foreruuner, UNSC, or Covenant.
Using a system of gas chambers Engineers are able to flaot in normal gravity atmospheres rather easliy. Engineers are often deployed midflight from Covenant dropships.
Engineers are pacifists by nature. They do not consider humans as enemies and are rarely seen on battlefields. The Engineers nature is so well know most marines will go out of their way to not kill them even if they are on the battlefield. Engineers even consider pest control offensive. While capable of violence in extreme circumstances an Engineer is often viewed as friendly and are often abused by other Covenant species for this reason. The only reported death from an Engineer came when a human was attacking a Grunt Deacon and the Engineer threw a rock in an effort to save his friend
Lighter than Some - The first Covenant to kill a human, Lighter than Some then built a plow out of spare parts to the Covenant dropship was a peace offering. This would then become the basis of the Brute Chopper and aided the war effort. Lighter than Some was ambushed and ripped apart by the Drones aboard the ship in revenge of the Engineer being assinged matainece duty of the ship. His best friend, the Grunt Deacon Dadab would get revenge by gunning down all the Drones onboard.
Vergil - Rogue Engineer that merged with the Superintendant personality in the New Mombasa city core. This Engineer defected to the humans during a mission lead by Captain Veronica Dare and shared information with Master Sergeant Avery Johnson about the Covenant battlenet.