Now that Master Chief and Blue Team have been captured by Cortana, now it's all upon the Fireteam Osiris to save the Blue Team from the Cryptum and end Cortana's plan with Guardians. Let's check out what happens in the Final Mission of Halo 5: Guardians.
You'll take control of Locke alongside Fireteam Osiris will now hunt for the Cryptum's cores to stop the Cryptum and free the Blue Team with Master Chief. Exuberant Witness will be guiding us to locate all five cores. So first head straight then take right towards the platform which will take you take you to a lower level.
Once you land on another section, keep moving forward until you encounter a bunch of enemies, Knights and pair of Hunters. Eliminate them all then Exuberant will guide you with the next section where more enemies are waiting, take all of them down. Keep moving forward through the next door and your objective will now is to eliminate all the five cores in order to retrieve the Blue Team.
The Alpha core is right in front of you so take it down along with the enemies around. Then use Artemis Tracking system to scan the area and locate nearby cores, you need to be quick so let the team take care of the enemies and you focus on the cores. You'll have a chance to fly Ghost and Mantis so use them wisely.
After eliminating all five cores, you need to reach the Cryptum and activate the access for Exuberant. On your way there you'll encounter a bunch of enemies as these is the last mission you bare with them, take them down as well to activate the override and pass through the next section.
Once you enter the Cryptum's facility, you'll face few more Prometheans and stop them until Exuberant cracks her master level in the Monitor. So start eliminating the enemies also make use of the Splinter Turret in the center to take them fast. Later Exuberant will ask you to activate two auxiliary stations for her. Head to the Upper Ledge, locate and activate the first Auxiliary Station then move opposite side of the room to locate and activate the second Auxiliary Station.
Now that the Auxiliary station is activated you'll have to deal with few more foes then Exuberant will open the Passage platform and you need to locate Cortana's Guardian which also has the Cryptum. Now it's just Locke, you need to keep moving forward when you'll be attacked by the energy relays. Once you've defeated the relay access cutscene will begin and do watch the Final conclusion to Halo 5: Guardians.