Chapter 10: The Dark Sector is the tenth and last chapter of the game Dark Sector, and the titular chapter.
The tenth chapter is very brief, consisting of just two fights against regular enemies and then the final battle against Mezner.
After the incident at the tracks leading into the Vault, kill the enemy soldiers. Use the shift ability to sneak behind them more easily.
In front of the Vault's main gate, climb the ladder up to the control booth. Press the switch in the booth to open the gate.
Follow the tracks to the staging area, and fight several mutants there. Press the switch to call the elevator, which will take some time to come. More mutants will also show up during this period, so keep fighting them off, using the crates for cover and using the shield and shift abilities for an additional edge. Once the elevator comes, ride it down to the lower level.
At the lower level, go through the open doorway to a large storage area. Press the switch in this storage area to open the locked door back near the elevator. Continue forward through the tunnels.
Mezner attacks by having several giant tentacles trying to shoot or squish Hayden. However, surviving the tentacles' attacks is nowhere as tough as actually getting rid of them, which will finally render the boss vulnerable. Ammo cases can be found along the corners of the area. For an easier time, use the electricity from the pillar above the boss to charge the utility canister on the rightside pillar, so it serves as an additional source of electricity, one that also won't be blocked by the motions of the tentacles.
During the first part, Hayden must shoot the three tentacles until they open up (when they spread wide their claws), and then he must use the electricity from the pillar or the utility canister to hit the middle points of the tentacles, or the bright spots between the open claws. Use After Touch to guide the glaive into those weak points if necessary. Once all the tentacles are defeated, the canopy over the boss will open, so hit him with the glaive or any other weapon.
During the second part, Hayden has to do the same thing to two tentacles, but an infinite flow of zombies will also come, making this part very difficult. Quickly gun down any zombies in the way, and then focus on blasting open the tentacles and electrocuting their middle spots. Again, when all the tentacles are down and the canopy opens, hit the boss.
During the third part, the zombies will stop appearing, but there will be five tentacles, two of which are on either side of the area. For an easier time, get rid of these two side tentacles first, preventing them from attacking from multiple directions at once. When the canopy opens, hit the boss for the third and final time to kill him.
If Hayden defeats Mezner, congratulations on finishing the game, and enjoy the ending.