The war in Halo: Campaign Evolved is not a simple two-sided fight. You crash onto a Forerunner ringworld caught between a desperate human military, a fanatical alien empire, and a parasite that wants to consume both. Knowing who each group is, what they want, and why they turn on one another is the key to following the story from the Pillar of Autumn all the way to the Maw.
Quick answer: The campaign revolves around four powers — the human UNSC, the alien Covenant, the parasitic Flood, and the long-gone Forerunners. The UNSC includes the ODSTs, the Office of Naval Intelligence, and the Spartan program, while the Covenant includes the Brutes and the new Sacristan Order introduced in Operation: METEORITE.
| Faction | Your relationship | What defines it |
|---|---|---|
| UNSC | Your side | Humanity’s combined military, fighting to survive the Covenant. |
| Covenant | Main enemy | A religious alliance of alien species that worships the Forerunners. |
| The Flood | Enemy of everyone | A parasite that infects and controls living bodies. |
| The Forerunners | Absent, but everywhere | Extinct builders of Halo whose technology drives the whole war. |
United Nations Space Command (UNSC)
The UNSC is the side you play for. It rolls the Navy, Marine Corps, Army, and Air Force into a single organization. Centuries before the story begins, humans left the Sol system and spread across the stars, and that expansion eventually put them on a collision course with the Covenant. By the time you take control of the Master Chief, the UNSC is fighting for its survival.
There is one detail worth remembering. The UNSC actually has five branches, not four. The four public branches operate under the United Earth Government, but the fifth, Spartan Operations, answers only to UNSC High Command. That alone tells you how secretive and elite the Spartan program is.
The Spartans and the Master Chief
The Master Chief is a Spartan, which is why he can shrug off punishment that would kill an ordinary soldier instantly. The Spartan program produced the finest troops humanity has ever fielded, and in Campaign Evolved you control their best.
Office of Naval Intelligence (ONI)
ONI works quietly in the background. You never fight beside them directly, yet their fingerprints are on nearly every major operation in the story. Their most notable plan is Operation: RED FLAG, a mission that showed how desperate humanity had become. The idea was to seize a Covenant warship, use it to find the Covenant homeworld, and capture one of the three Prophets in the hope of forcing peace talks.
Orbital Drop Shock Troopers (ODSTs)
The ODSTs are the toughest of the UNSC’s regular soldiers. They drop straight into combat from orbit inside small pods, often landing right in the middle of a firefight. Unlike Spartans, they have no superhuman abilities and no powered armor. They rely on training, skill, and sheer nerve.
That difference feeds a long-running rivalry. Many ODSTs feel Spartans take the credit thanks to their strength and high-tech gear, while regular troops risk their lives in the same battles without those advantages. The tension has simmered between the two groups for years.
The Covenant
The Covenant is the enemy you spend most of the campaign fighting. It is a massive alliance of different alien species bound together by a shared religion. They worship the ancient Forerunners as gods and treat their relics as sacred.
The Covenant also fights very differently from the UNSC. Its military splits into separate groups that handle everything from enormous warships to ground infantry, and the Special Operations Division fields some of its strongest and most skilled warriors. Their weapons are alien too, firing plasma instead of bullets and using technology humans still do not fully understand. In practice, Covenant plasma weapons are excellent at draining your energy shields, so it helps to swap between human and Covenant guns depending on who you are facing.
The Brutes (Jiralhanae)
The Brutes, or Jiralhanae, are one of the strongest species in the Covenant. They joined the alliance much later than the others but quickly became some of its most feared soldiers. They favor raw strength and an aggressive style, charging straight in and hitting hardest at close range.
The Sacristan Order
The Sacristan Order is a special Covenant group that appears in the Operation: METEORITE prequel missions. Unlike standard Covenant forces, it is led by a Prophet called the Minister of Harmony, who is obsessed with ancient Forerunner technology and runs genetic experiments on Brutes aboard a secret research vessel. You can spot the order by its distinctive white, gold, and purple armor, decorative robes, and ceremonial designs that make it look more like a religious sect than a normal army.
The Flood
The Flood is the campaign’s biggest twist. At first it seems like a straight fight against the Covenant, and then the Flood appears and reshapes the entire story. It is not an army with soldiers and commanders. It is a parasite that infects living creatures and takes over their bodies, turning every victim into part of itself and spreading faster with each new host.
What makes the Flood terrifying is the sheer number of them. They swarm and surround you quickly, and watching enemies you fought moments earlier rise again as infected forms makes them one of the most unsettling threats in the game. In Campaign Evolved, the Flood roster is expanded with new enemy types, including the pure forms first seen in Halo 3.
What is a Gravemind?
As the Flood infects more creatures, it eventually forms a Gravemind, its central intelligence. Instead of behaving like mindless monsters, the infected merge into a single hive mind that can think, plan, and coordinate the Flood as one force. You do not meet a Gravemind in this campaign, but it remains a defining part of the Flood’s role across the wider Halo series.
The Forerunners
You never actually fight the Forerunners. By the time the story starts, they have been gone for roughly 100,000 years. Even so, they sit at the center of everything, because almost the entire conflict revolves around what they left behind.
The Forerunners were a hugely advanced civilization that built astonishing technology, including the Halo rings themselves. Their ruins, weapons, and machines are scattered across the galaxy, and both humanity and the Covenant are racing to unlock their secrets. As you play, you move through Forerunner structures and systems constantly, and that legacy shapes what happens throughout the series.
How the factions turn on each other
These groups do not only fight you. They fight each other. Once the Flood breaks loose, it stops caring about species or loyalty, and both the UNSC and the Covenant become targets in equal measure.
That shift creates moments where you are effectively fighting alongside Covenant forces against a shared threat, even though they will happily kill you again the second the Flood is handled. It is a strong dynamic, and it keeps the campaign from ever settling into a plain two-army war. Understanding that each faction chases its own agenda, and that those agendas keep colliding, is what makes the ringworld feel like a real war rather than a string of separate fights.
