The Swarm was a vicious extragalactic insectoid species that invaded the galaxy in 2733. The subsequent war cost the lives of over 300 trillion Therans and destroyed or devastated over 20 million worlds of The Royal Allegiance alone, and far more when including losses from the Sangheili, UNSC, New Liberty, and The Order.
The Swarm are speculated to have invaded the galaxy once before, long after the time of the Forerunners. Based on unsubstantiated evidence provided by an unidentified source on Thera, which may or may not be a Forerunner AI, the Swarm invaded the world of Thera. According to the source, it defended the world, along with the Therans, who were technically descended from Forerunners, with 'fire and vengeance'. It is not known if this event was related to the Ancient War, an apocalyptic event before the de facto founding of The Allegiance. The only evidence for it was carvings in ancient temples and archaic writings in obscure, long-dead languages.
The first confirmed contact with The Swarm was in 2614, when they invaded and massacred the small Allegiance scientific research outpost of Radiance. Due to the infrequent nature of communications between the outpost and the rest of the Allegiance, its destruction was not known until 9 months later. After this event they vanished off the galactic radar for more than a hundred years, before reappearing in 2733, triggering the galaxy's most massive, bloody and costly war since the Forerunner-Flood conflict.
Although it was first thought that the Swarm was a single species, it has been theorized that is several different races in a collective, similar to the Covenant, and possibly controlled by a more advanced race as the brains, the many forms just acting as the others brawn.
Despite the fact that The Swarm was one single species, there were several different 'forms'.
Hivelings are the youngest of all the beings in the Swarm collective, coming in many shapes for each race. They all look like a small egg-shaped baby, although as they grow this protective carapace shatters and they look nothing alike.
Drones were the most simple and most numerous of the Swarm forms. They were more intelligent than the other forms, possessing knowledge and learning skills just like a human or Theran would. Their average height was around 7 foot, though they could grow as large as 8 foot before the end of their lifespans. They were stronger than the average human and possessed extraordinary endurance and resilience. Their faces were insectoid and their shape human-like. They possessed enough dexterity to wield most weapons in battle and could hold their own during combat situations. They were born already 6 feet tall and ready to fight, and continued to grow until the end of their lifespans which generally lasted about 20 years. They possessed tough exoskeletons which acted as natural armour to a certain degree, though it generally protected less from blades and as a result bladed weapons were popular throughout the war. Drones had to shed their exoskeletons every three to four years as they outgrew them. In actual fact, Drones were the larval form of Harpies, which they transformed into after about two decades of life. The older a Drone was, the more powerful it was and the higher it ranked in terms of military.
The adult form of the Drone, Harpies were much stronger and more potent however they lost the vast majority of the intelligence they possessed as Drones. They were used akin to air support, also as rapid melee units to quickly engage enemy forces. They could easily pick up a fully armoured soldier and carry them to their deaths or feast on them on the wing. In large swarms they even posed threat to armoured vehicles and serious danger to less maneuvrable ground support vehicles, a notable example being the AV-14 Hornet. Armed with extremely powerful wings and razor-sharp blade-like legs, the sight of Harpies on the horizon often heralded the end for ill-equipped infantry. They were, however, quite weak against anti-air batteries, SAMs and more rapid air vehicles.
A powerful Swarm type, the Constrictors were widespread and deadly, often used as war mounts and heavy infantry against almost anything on the battlefield. They were highly feared as a rapid threat able to engage infantry formations or armoured units equally effectively.
Swarm Tarantula's are the so far known largest of the Swarm beasts, and fight on almost all fronts except space warfare. Although they seem unstoppable, seasoned Allegiance units have learned to take it down by firing at its belly until its disembowled, as that is its weakest point of the shell.
Clackers are small and agile insect-like creatures, about the size of a child and acting in a ground support role for Drones. With vicious claws and terrific bite, Clackers are so named for the chomping noise of their pinchers and the horrible crunch enemies make when they are torn limb from limb.