Cleansing is the eleventh level of the RTS game, Halo Wars. This is the first level you can control Cyclops, and second level to control the Gremlin. It takes place on the hull of the UNSC Spirit of Fire.
After the UNSC Spirit of Fire is pulled beneath the surface of an unknown planet, it is revealed that there are Flood on the ship and Sergeant John Forge is tasked with cleaning them off. Very soon, Sentinels appear to clean off the Flood, and deem the UNSC a threat. The Spirit of Fire's A.I. Serina intercepted an alien signal noting that they needed to eliminate the Flood off the ship, and a "Cleansing Ring" is needed for doing that job. Sergeant Forge, along with Marine squads, Red Team Spartan IIs, Wolverines, Hornets and Turrets, kill and destroy all the Flood hijackers.
After that, they proceed to disable all other Flood forms that are stuck onto the ship, but only a Cleansing Ring will permanently kill the Flood. Super Sentinels soon arrive to clean off the Flood. When the cleansing ring approaches, Serina orders all ground units including the Sergeant into airlocks, while she advises the Hornets to head the opposite way the cleansing ring is approaching so that damage to the air vehicles is minimized. They eventually succeed to clear every Flood form on top and Captain Cutter orders the ship to get out of the tunnel they are in.
Cutscene opens to the Spirit of Fire going through the split seen in the last level. Sentinels continue to fly around the Spirit. One flies in front of the bridge and observes Captain Cutter before flying off.
- Serina: This is all part of the plan... right, guys?
The ship continues to fly through what appears to be a massive underground Forerunner structure.
- Cutter: Serina, report! What's going on here?
Two Sentinels fly towards where the Spartans, Forge, and some Marines fight off Flood. Flood dispersal pods start dropping everywhere, one of them hits one of the Sentinels.
- Serina: "Best guess, we're being pulled through some sort of docking system. That alien life form followed us in, we have a significant infestation on the aft quarter."
- Cutter: "Forge, get out there and get those things off my ship! You can requisition troops from the airlocks on the deck, if we lose those airlocks, we'll be defenseless."
More Sentinels fly in, as well as a couple of Protectors being lead in groups by Sentinels.
- Serina: "Those aliens are dropping pods on the hull from all around us, keep watch for them."
- Serina: "Alien population on the hull is increasing, and the structure is reacting, more of those drones inbound too."
After several Sentinels have been destroyed:
- Forerunner AI: "Security barrier breached. High order infection detected. Unidentified alien intrusion detected. Initiating Stage 3 Defense Protocols."
- Serina: "Bad news, I intercepted a signal from the alien AI. Those drones have declared us a threat and are massing behind us."
- Forerunner AI: "Decontamination sequence commencing."
- Serina: "Sergeant Forge, get your men into the airlocks. Something very bad is coming."
A ring-shaped energy field starts up clearing away the Flood from the Spirit of Fire.
- Serina: "The energy field is reacting to the aliens, it's trying to clean our hull, take cover out there!"
- Serina: "Captain, if our air units fly through the ring in the opposite direction it will minimize damage."
- Serina: "The drones and the cleansing ring seem to be able to remove the aliens if they're already damaged."
Every time you pass through a ring a countdown begins to show how much time you have to the next ring. This will continue until all the Flood is cleared off the ship, every time you approach a ring the Forerunner AI will talk about decontamination and Forge or Serina will warn you about the ring or to take cover :
- Forerunner AI: "Decontamination sequence commencing."
- Forge: (second ring) Take cover! Another cleansing wave is coming."
- Forge: (third ring) "The ring is back!"
- Serina: (fourth ring) "Take cover!"
- Cutter: (after third ring) "Two Vultures have just finished repairs, they're yours to command."
When enough Sentinels are killed, Super Sentinels will show up to help the Regular Sentinels and Protector Sentinels.
Once all Flood forces is cleared away:
- Forge: "All clear! The hull is clean."
- Cutter: "Serina, maximum power to the engines. Get us out of here!"
- This level introduces a miniature Halo Array, except rather than having an effect range on a stellar scale, the ring only creates an energy field within its own diameter, destroying any form of life, including Flood. The energy field also damages other technologies, such as vehicles, buildings, and turrets. While entering the insides of the planet, the crew aboard the Spirit of Fire must travel through a narrow tunnel through the thin crust of the Shield World's outer sphere. The miniature Halo Array "cleanses" any Flood or unknown sentient species, protecting the interior of the Shield World. Nonetheless, the Flood have apparently found a way to bypass this quarantine system, and have successfully taken hold on the Shield World's interior, as is revealed in the next level, Repairs.
- Near the start, Sentinels are shown as yellow blips on your Radar, but after the first ring comes, they are shown as red blips.
- On closer inspection, one will notice that the whole level is an optical illusion, as the Spirit of Fire is not actually moving, the walls are. This can be seen properly if one moves the camera up against the tunnel walls and looks at the Spirit.