Zero Hour

Zero Hour was a mission in Destiny 2, which you had to complete to obtain Outbreak Perfected, an exotic pulse rifle, which is a remaster of Outbreak Prime, a SIVA enhanced pulse rifle from Destiny. Zero Hour was timed, similar to The Whisper. You are met by Mithrax, the Forsaken, a Fallen Captain who you become allies with, after you spare him in the 'Enemy of my enemy' quest on Titan.

Guide
At the start of the mission, you head through some half-opened doors in the Tower Ruins. Walk through those and Mithrax spawns in, and kills a Dreg stood in your way. Advance through further through a small ventilation system to enter further into the tower ruins, where The Speaker usually stands. Kill all the fallen and move into the next area. In the next area, there is the vaults, and a blocked off entrance into the hall of Guardians. It is infested with Fallen so you must kill those to advance into the next area. Move towards the Tower Hangar and continue killing the Fallen, this time a Walker will spawn. After this go through a doorway and kill more Fallen. After you kill all of the Fallen, move through down an elevator shaft and through a vent into the next area.

Where to go
On your left when you leave the vent, there is a ship, and underneath it, is a little opening for you to walk down. Go through there and to the very end and to the right, there is another vent for you to go through. After this, fall down and go to the very end of this small hallway, jump up and take a left, where there will be another hidden vent for you to crawl through. Go through that and take your first right, and fall down onto the platform directly beneath you. Be careful though as you may take fall damage and the platform will break off, so if you fail, don't forget to tap jump before you go through. Follow the vent through until you meet a platform outside, and hop onto it. Jump all the way down and hug the wall until it ends, and hover over to the platform. Pull the lever and walk up the stairs, and there will be platforms above you which you must jump onto. There will be an open hatch, so jump up an into more vents. You will fall down into a room where you must go to the top right vent and open it. Go through until you go into a massive room with spinning blades underneath. Evade the spinning blades by going through the gaps lit up with red. By the way you can also stand on the middle of these fans/blades. Once you reach the bottom, go through the vents and turn left until you get to an empty room with small ledges on the side. Jump on those and head to the end and pull a lever for your teammates to get through easier. Fall down through some more vents and you will enter a room with some panels and a screen with a maze layout and some red lights.

TR3-VR
Go down through some more vents and you will enter the maze, patrolled by an Arc emitting bot, called TR3-VR. There are little cubby holes for the player to hide in to not get 1-shotted by this thing. You must go through the maze and hit all 4 levers to get to the next section. You will know TR3-VR is coming by the loud robotic noises and the red lights that appear when he is nearby. Once you get to the end there is an elevator. Go up it and at the top there is a vent.

Where to go next
After that, go down the slide and avoid hitting the objects, then take a left, then a right into a vent. Once you are in the vault, turn to the left and walk into the Cryptarch symbol. A door will open, and head through a massive hallway to the end and take a right, drop down and go up and take a left. Then jump up through a vent and drop into the boss room

Boss Fight
This fight, you encounter Siriks, Loyal to Eramis. There will be 2 large Servitors, named Mimiks-2, Loyal to Eramis, which spawn Shanks and two Walkers that spawn as you weaken the boss, along with the turrets, shanks, vandals, marauders and dregs. Once you kill everything, you are done and will be rewarded with Outbreak Perfected.