It’s mind boggling that Bungie was able to fit in so many epic moments into a single chapter of Halo 3. The Covenant is by and large the best Halo mission in the series, containing within it multiple groundbreaking firefights, intense plot development, and top tier vehicular combat.
The game developers really went all in on this one. It features one of the most intense beachhead assaults in the series, that included many vehicles but especially the Hornet. Racing to stop the Prophet of Truth is a hell of a ride, made further complicated by the introduction of Flood to the Ark and a double Scarab showdown before the main gates. At no other point in the series is the player allied with the Flood, which is a strange experience but really awesome regardless.
Significant characters perish during a critical rescue, creating intense waves of drama in the narrative. Finally, the episode is capped off with the astonishing revelation that the Ark is actually a Halo ring production facility, and the only way to stop the Flood is to fire off the newly constructed ring.
This requires retrieving Cortana from the infected bowels of High Charity, which taper off the mission with a foreboding sense of danger. All of this underscored by the fantastic orchestral blessings by Martin O’Donnell and Michael Salvatori.