Cleansing

Overview
Cleansing is the eleventh level of the real-time strategy game, Halo Wars. After the UNSC Spirit of Fire is pulled into the underground 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-like 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 Spartans, 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 can be permanently killed by the cleansing ring. 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.

Trivia

 * This is the first level to introduce Sentinels, due to the fact that the level's skull is the first one to have as an objective to destroy Sentinels.

Skull
To get the skull the player must destroy 100 sentinels, the skull spawns at the bow of the ship and is only possible to get via hornet due to it being under the command post