From Halopedia, the Halo wiki
Acting on Spartan Griffin's information, John-117 and the Weapon investigated Site Novem, a Banished excavation site. At the seemingly deserted dig site, they found that the Banished's mining laser had cut through Forerunner columns into a subterranean structure. Before they could enter, the mining laser activated, preventing the Spartan from entering the tunnel. After analysis by the Weapon, John-117 destroyed the mining laser's cooling pylons. When he returned to the mining laser's control room, he was met by the dig site security commander Bassus. The two fought, with the Spartan emerging victorious. John-117 shut down the laser, and prepared to enter the tunnel.
If the Chief stalls or does some other activity:
The Chief heads north towards the dig site. He breaks through a Banished blockade guarding a recently established bridge.
The Chief approaches the dig site, which has been established at the base of a mountain ridge. The area around it is completely barren, the Banished having stripped the land of all natural resources.
The Chief enters the dig site and makes his way down the crater. Strangely, there are no Banished troops present.
The Chief approaches the tunnel, which goes underneath the ridge itself. The Weapon's avatar appears in his hand.
She turns around.
Her avatar vanishes. The Chief turns around and looks up to see a massive mining laser mounted at the top of the facility powering itself up and being aimed in the direction of the tunnel. The Chief dodges out of the way as the mining laser shoots four powerful beams into the tunnel, effectively blocking the entrance to the Conservatory.
Banished drop pods are deployed, releasing multiple Brutes.
The Chief engages the Brutes.
More drops pods are deployed. A Banished Phantom arrives and delivers even more hostile reinforcements.
The Chief fights his way to one of the central structure's gravity lifts and rides it into the mining laser. Once inside, he engages the laser's crew.
The Chief eliminates the crew.
The Chief heads over to the terminal by an energy barrier at the front of the structure. He deploys the Weapon onto the terminal.
The Weapon stares at the laser beams blocking the tunnel.
The terminal beeps but nothing else happens.
John uploads the Weapon back into his armor.
A ramp at the back of the central structure lowers itself, leading outside. The Chief heads down the ramp and starts searching for his new targets. Phantoms and drop pods arrive with more enemy reinforcements that he has to deal with.
The Chief exposes the cooling pylons for one regulator.
The Chief destroys both cooling pylons. The regulator explodes.
The Chief exposes and destroys the cooling pylons for the other regulator, causing it to blow up as well.
The Chief goes back inside the central structure and heads to the controls again.
Master Chief's POV: The Chief starts to deploy the Weapon when something grabs him by the neck and lifts him off his feet. John finds himself face-to-face with an aged but fearsome-looking Jiralhanae Chieftain. The Brute chuckles menacingly and roars in his face, before flinging the Chief across the room. The Chief lands on his back. He sees the Chieftain approaching, gravity hammer in hand.
The Chief uses his Grappleshot to pull a container towards him.
Bassus raises his hammer.
The Chief uses the Grappleshot to swing the container around and strike Bassus on the side of the head, knocking the Brute off balance and causing his hammer to miss. The shockwave from the gravity hammer pushes John away from Bassus. The Chief gets back on his feet and faces the Chieftain.
For Bassus' dialogue during the fight, see here.
The central structure goes into lockdown.
Even by a Brute's standards, Bassus is incredibly strong and durable. It takes a lot of ammunition, but the Chief finally brings the Chieftain down.
If the Chief manages to kill Bassus quickly:
The Chief returns to the control terminal and deploys the Weapon.
She cracks her knuckles.
She starts hacking the terminal.
She snaps her fingers and the mining laser explodes.
The Chief retrieves the Weapon.
The central structure's lockdown is lifted.
The Master Chief leaves the facility and enters the tunnel.
The following achievements can be unlocked on Excavation Site across the all editions of Halo Infinite.