The next step once you finish up Tier 3 is to make your way to the Van in New York. Hop into the back of the van(there may or may not be a man offering you candy driving it) and stand there for a few seconds. You should be taken to a loading screen and wind up in a warehouse.
Once you’re in the warehouse, move to where you see the below opening in the boxes. Make your way in through there and let the maze crawling begin. A few tips, Jump up boxes as much as possible. Being higher up lets you vault over rows of boxes, and also lets you see where you are, and where you’re going. Make sure to stay aware of which direction you started from.
I got lost a few times, so I don’t have screens for exactly where to jump and when, but your goal looks like this… I ended up against the left (left from starting point) wall on the very top of the boxes looking in towards the center. You see the glowy light? Head over to it, that’s where your dead agents are. Remove the spinal chip from the one that glows before moving on.
You will eventually come across an Egyptian Dispatcher. Start killing it, it will eventually disappear somewhere, leaving you with a debuff that will supposedly kill you in 1 shot. As I’m a tank and have quite a bit of health, it only took out 25% of my hp. It may or may not kill you, let me know in the comments below =)
After the cut-scene, head over to Agartha (The World Tree) and then go and begin your adventures in Egypt.