The Walls Come Down/Walkthrough
From Destinypedia, the Destiny wiki
After the cutscene, you will find a giant Ketch and a destroyed wall in front of you. As you start moving towards the marked location, you'll encounter a good number of enemies nearby guarding the wall. As these are Level 42 enemies, we can easily take them down one by one. If you still having trouble then try taking cover behind the rocks to regenerate some health.
Get inside the Broken Bridge and get ready to face some more enemies all the way through the tunnel. Move slowly as the Dreg are very smart and hide behind the walls, either use grenades to shot a fire to make them jump out. Take them down and head towards the second tunnel which is the exit towards the new area - The Divide.
Investigate the Pattern
As we see there are no signs of Sepiks and it's all surrounded by the Fallens. Try to Scan the pattern which is a Red wire thing which is consuming everything around it just like SIVA. You must find the root which is the sensor grid, but first, you need to deal with a Hive which will soon drop by and start spawning the Fallens. Along with regular Dregs and Vandals now there are Wizards and Acolyte joining their clan.
Activate the Sensors and Eliminate the Fallen
Once you Activate the Sensors, quickly climb up the stairs behind to take cover. The Fallen now spawning will be augmented and even Lord Saladin has no idea about the corrupted enemies. Defeat all the mutated Fallens inside the Monitoring room to complete the mission. You will be facing Splicer Vandal, Splicer Captain, and Splicer Dreg which are more powerful and upgraded.
As soon as you Defeat all the augmented Fallens and let Ghost collect all the data from the Grid. Thus completing the mission and continue to the next chapter - The Iron Temple and The Plaguelands.