Were you looking for the second chapter in the episode "Key"?
UNSC Marines request Spartan assistance after failing to neutralize a Covenant stronghold as well as Forerunner anti-air turrets. Commander Palmer responds by sending Fireteam Crimson to seize control of the area. Crimson engage the Covenant, mostly consisting of Unggoy and Kig-Yar warriors. Upon getting closer to the structure, Crimson encounters Promethean Watchers. After clearing all Covenant in the area, Crimson is directed by Jared Miller to two pylons powering the Forerunner turrets. In the process of disabling the pylons, Crimson fights off Promethean Crawlers supported by more Watchers.
After disabling the pylons, the turrets are deactivated. Before Crimson is extracted however, Miller picks up a slipspace anomaly nearby. Crimson is ordered to investigate. They encounter more Promethean forces, including multiple Knights, who translocate into the area. Crimson is forced to clear the area again. After doing so, Robert Dalton directs a Pelican dropship for Crimson's extraction. As the Pelican arrives, Miller receives a distress call from a Science team who request for UNSC assistance. Palmer acknowledges and immediately sends Fireteam Crimson on the way.
Note: All dialogue is spoken through the COM.
After the player reaches the top of the structure and kills all of the Covenant.
After the player kills several Watchers.
The player kills the last Watcher.
Two waypoints appear at either side of the structure's base.
The player approaches the first waypoint several Crawlers and Watchers appear.
The player activates one switch.
As the player nears the second waypoint several more Crawlers and Watchers appear.
The player activates the second switch.
A waypoint appears near the front of the structure. More Watchers and Crawlers appear.
Four ordnance pods containing two rocket launchers, a railgun and a SAW drop at the base of the structure. As the player approaches the pods several Promethean Knights appear.
More Knights phase in.
After the player kills most of the Prometheans.
All Prometheans are neutralized:
A Pelican approaches and hovers in front of the structure.