Map Legend: [B - Background Info]   [C - Credits]   [M - Medi-Gel]   [P - Power Cells] [R - Resource]   [U - Upgrade]   [W - Weapon]

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Background

After finishing your first batch of dossier missions -- Archangel, The Convict, The Professor, and The Warlord -- Yeoman Chambers will let you know that the Illusive Man would like to meet with you in the briefing room. When you talk to him there (by activating the message terminal), you'll learn that the human colony at Horizon "just went silent," and so if you're quick, you might be able to catch the Collectors there. The Illusive Man will also mention that one of your former crewmates -- either Ashley Williams or Kaidan Alenko, depending on who survived at Virmire in Mass Effect -- is stationed at Horizon, and that it might not be a coincidence.


Walkthrough

At Horizon, you'll be attacked by a series of Collectors and husks, and you'll recognize that these new husks are more "evolved" than the ones you faced previously. You'll also come across some frozen colonists, which will explain how the Collectors were able to abduct humans without leaving behind any signs of struggle. Fortunately for you, Mordin's research will keep you protected from the Collectors' seeker swarms, and so you won't have to worry about sharing the same fate as the colonists.

When you reach Park 2, the Reaper ship Harbinger will start taking possession of regular Collectors, making them more powerful. The best way to deal with these possessions is to kill all of the regular Collectors first, and then concentrate on the possessed Collector. Otherwise, you might have to kill possessed Collectors multiple times. Also, when Harbinger takes control of a Collector, the Collector will float in the air in plain sight for a while, giving you a chance to score some easy damage. Always take advantage of situations like that.

When you reach the Power Station, you'll meet up with a mechanic. He'll tell you that Horizon has "high powered GARDIAN lasers," but that their targeting system is messed up and needs to be calibrated. You'll then decide that the lasers might be the best way to drive off Harbinger.

Past the Power Station, you'll come to the Spaceport, which you'll find defended by two scions and some husks. The scions will start out in the middle of the area, and the husks will try to flank you from the north and south. The husks shouldn't pose much of a problem, but the scions can be nasty. To defeat the scions, just realize that they're slow moving and slow firing, and that they never try to take cover. So just hide until they attack, then jump up and take a few shots at them, and then hide again, and repeat until they're dead.

After the battle, you'll need to get the laser targeting system on line. To do this, you'll need to hack the computer in the middle of the Spaceport. This will allow you to speak with EDI on board the Normandy, and she'll then get to work on calibrating the lasers. However, while she's working, waves of Collectors will fly in and attack you. After each wave, you'll get a chance to scavenge for some bullets. After the second wave, a huge creature called the Praetorian will fly in.

The best way to defeat the Praetorian is to use heavy weapons against it -- specifically, the Collector Particle Beam that you should have picked up during the mission (from Pathway 2). The Preatorian will focus on you rather than your crewmates, so you'll have to do some running around to maintain your cover. Other than that, you'll just need to fight the Praetorian like you would most other enemies: take shots when it's safe, and hide when it isn't.

When you take down the Praetorian, EDI will gain control of the lasers, and she'll drive off the Harbinger. That will free the remaining colonists from the seeker swarms. Kaiden / Ashley will then show up, but the reunion won't go very well, and he / she won't feel inclined to join your crew, even if he / she was your romance option from Mass Effect.

Back on board the Normandy, the Illusive Man will tell you that your next target should be the Collector homeworld past the Omega 4 relay. He still won't have a way to get you through the relay, so he'll recommend that in the meantime you continue to build up your team, and prepare them for what might end up being a suicide mission. To that end, he'll give you three more dossier missions (for Tali, The Assassin, and The Justicar), and you'll also be allowed to complete some loyalty missions (starting with Miranda and Jacob). Yeoman Chambers will let you know when loyalty missions become available.


Rewards

• Experience: 1000
  
  
• Credits: 60,000 (6000 from datapad in Loading Zone, 4200 from datapad in Home 1, 4800 from safe in Home 3, 3000 from computer in Home 4, 3000 from computer in Home 5, 6000 from wall safe in Power Station, 3000 from computer in upper level of Commons 2, and 30,000 from Cerberus)
  
  
• Resources: 2000 Platinum (from container in Power Station)
  
  
• Upgrades: Heavy Skin Weave (from dead Collector in Park 1) and Biotic Damage (from dead Collector in Commons 2)
  
  
• Weapons: Collector Particle Beam (from particle beam in Pathway 2)