From Halopedia, the Halo wiki
In 2558, while attempting to secure "Apex" near the end of the Requiem Campaign, Forest's Pelican was shot down. Soon after, Fireteam Crimson arrived and the two fireteams joined forces to retrieve a new power supply for a Covenant Harvester. Working together, Crimson and Forest eliminated heavy Covenant resistance, including forces deployed from a Covenant Lich. After the Lich moved to a more vulnerable position, Crimson boarded the craft and stole its power supply to replace the Harvester's. The two fireteams then used the available vehicles to race to the entrance to the "Warrens" to get extracted by a Pelican to get the power supply back to the Harvester.
Unlike other SPARTAN-IV NPCs in the campaign and Spartan Ops, the members of Fireteam Forest are invincible. Like the non-playable members of NOBLE Team in Halo: Reach, their shields can be disabled, though they will recharge at the normal rate. Their vehicles, however, are not invincible, and the Spartans can be killed if they fall off a cliff. Given their durability and competent AI, Fireteam Forest can eventually eliminate all enemy forces, such that the player(s) need only perform contextual events when prompted.
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