.3J.01. Horizon: Investigate

After completing all given Recruit missions and speaking with the Illusive Man, you are sent to Horizon to investigate. You'll encounter your first Collectors here.

Collectors are organic. At any given time, one of the Drones will be possessed by the Harbinger, granting the drone Shield and Armor, although it has no health. If you kill the Harbinger, it simply possesses another Drone nearby in a few seconds. One strategy is to take out as many Drones as possible before dealing with the Harbinger. You can also focus on the Harbinger, as his "possession" animation of a Drone lasts several seconds. In this time, you can blast him full-on with any abilities and guns that you have, although usually this is only enough to get his barrier down.

You won't face the Harbinger until you get a little ways in. You will face several Collectors. The assassins will use a particle beam that's pretty harmful to you. The colony offers tons of cover, so use that to your advantage as you take on the groups of 4-5 Collectors in each area.

This map is fairly linear, although items are usually hidden inside the residence units. Horizon is a large colony, so you'll do a lot of walking. Keep clearing the areas of the Collectors and keep moving on each time. The Collectors never really surprise you, flying in from above.

You'll encounter some frozen colonists and more items before the next fight. Make sure to pick up the Collector Particle Beam, which is a decent laser weapon with much more ammo. It has great accuracy, no recoil, and it never needs to be reloaded.

Going a bit further past this area will introduce you to the Collector General, Harbinger. The collector pods can be used for cover if you need, as Harbinger's missile-seeking weapon will knock you out of cover if it hits close to you.

When you clear the area, check the residences for items, then bypass the door to continue.

You'll discover an unfrozen human survivor after the door. Talking to him won't enlighten very much, but you do get a new objective: fix the defense tower targeting systems.