Horizon

Start Area Normandy SR-2
Mission Trigger Complete the Archangel, The Professor, The Convict, and The Warlord missions
Experience Reward 1,000 experience points
Monetary Reward 30,000 credits
Credits Found 27,000 credits
Platinum Found 2,000 platinum
After you finish the first four Dossier Missions, you will automatically be sent to Horizon for this Mission. You can't put it off, so if you really want to finish a lot of optional assignments beforehand, keep that in mind.

You will fight the Collectors for the first time on Horizon, and they have a whole array of different ways to annoy you. First, they all have barriers instead of shields. The same Weapons (i.e., SMGs) work well against them, but most Powers won't. The exception is Warp, which is fantastic since it also hits armor. Miranda is a valuable asset on this Mission.

Standard Collectors are no big deal at first, especially Drones. However, eventually Harbinger will show up, and it is a badass. With armor instead of health, it is immune to most Powers at all times, and it can take you or a party member down in seconds. Worst of all, if you kill it, it can possess another Collector (though this doesn't seem to be a frequent occurence on Horizon).

You'll also find Husks, which are a pain. Powerful Weapons like a Sniper Rifle or Shotgun may be able to saw them in half, so aim for the crotch rather than the head. Any biotics that lift them, such as Pull, will also kill them, but they have armor (on higher difficulties) so you need to break that first. They aren't tough, but they like to swarm you. Taking them out from afar is best, but it's not always easy.

Scions are my least favorite Enemy in the game. They have just one health bar, armor, but it takes a long time to whittle it down. Their energy pulse attack is devastatingly powerful, though easy to avoid if you see it coming and keep moving laterally. The problem is that doing this while dealing with Husks or another Scion can be really hard.

And finally there is the boss of this area, the Praetorian. Its base health is armor, but it will periodically add a Barrier on top of that. Its lasers are powerful, and when it drops from the sky it causes a large explosion in an area. Your best bet is probably to use the new Collector Particle Beam to kill it as quickly as possible. It won't be easy.
Compare Horizon to other Missions
Areas: Involved in Horizon – 1 shown
The planet or satellite where this area is located
Planet
The name of the area; most areas will be shown with their planet name, but some have no specific area name; DLC areas are only available if you have installed the relevant content
Name
Horizon Horizon
Enemies: Encountered during Horizon – 7 shown
The in-game name of the enemy; there may be different versions of an enemy, for example of different races, that share the same name; advanced enemies have more than two different health levels; DLC enemies are only encountered if you have installed the relevant content
Name
Some powers only affect organic or synthetic targets
Type
The different health bars this enemy has, in order from outermost to innermost; enemies may have less health bars on lower difficulty levels; each health bar type is affected differently by weapons and powers
Health Levels
Collector Assassin Organic Barrier, Health
Collector Drone Organic Barrier, Health
Collector Guardian Organic Barrier, Health
Harbinger Organic Barrier, Armor
Husk Organic Armor, Health
Praetorian Organic Barrier, Armor
Scion Organic Armor
Weapons: Found during Horizon – 1 shown
The in-game name of the base weapon; weapon upgrades will change each weapon's code and number; heavy weapons are used regardless of class and only be Shepard; exclusive weapons may only be used by a given character, and possibly Shepard under certain circumstances; DLC weapons are only available if you have installed the relevant content
Name
The class of this weapon; whether a character can use a particular weapon depends on whether they are able to use any weapons of that class; only Shepard can use heavy weapons
Type
Some weapons may only be used by certain characters; heavy weapons are all exclusive to Shepard; Shepard may use any unique weapon by choosing it at a certain point in the game (including those normally unique to other characters); however, you may only choose a weapon if Shepard is already proficient with that class of weapon
Exclusive To
The style of fire of this weapon; note that semiautomatic means the weapon will fire repeatedly until the clip runs out, while single shot means there is a significant delay between shots
Shot Type
The default number of shots you can take with this weapon before reloading; upgrades may increase this value; heavy weapons do not use clips
Clip Size
The default amount of reserve ammunition you can hold for this weapon; upgrades may increase this value
Reserve Ammo
Collector Particle Beam Heavy Weapon Shepard Continuous fire - 500
Upgrades: Found during Horizon – 2 shown
The name of the upgrade; multiple upgrades of the same name are cumulative; exclusive upgrades affect a single party member other than Shepard; ship enhancements are special upgrades that are used during the end game sequence; DLC upgrades are only available if you have installed the relevant content
Name
The type of upgrade this is, as shown on the research menu
Type
Who or what this upgrade benefits; "Squad" benefits apply to all party members, including Shepard
Benefits
The amount of credits or minerals required to buy or research this upgrade; for upgrades that can be repeated, the cost is multiplied by the number of the upgrade being researched; thus if an upgrade costs Level × 500 Element Zero, the second upgrade would cost 1,000 Element Zero, the third 1,500 Element Zero, and so on; purchased upgrades are not counted for this purpose, so the cost is the same whether purchased upgrades are purchased before or after other research; note that it is possible to get a 1/6 discount on the price of most purchased upgrades (see shop page for details)
Cost
Indicates whether the upgrade is purchased with credits or researched with element zero or minerals; upgrades that are purchased with credits are applied immediately, while others must be found and later researched on the ship
Currency
Biotic Damage Armor Upgrade Squad Level × 500 Element Zero
Heavy Skin Weave Prototype Shepard Level × 2,500 Palladium
Found Items: During Horizon – 18 shown
The area where this object is found
Area
The type of object to be examined
Examine
If the object requires a hack or bypass minigame to open, that is indicated here
Lock
The item found by examining this object; names in gray italics indicate items which are only available through downloadable content
Found
Where within this area the object can be found; these are normally listed in general order of appearance
Located
Horizon Power Cells - Heavy Weapon Ammo First area with collectors
Horizon Datapad Hack 6,000 credits First area with collectors
Horizon Dead Collector - Heavy Skin Weave Second area with collectors
Horizon Datapad - 4,200 credits Building near second area with collectors
Horizon Safe Bypass 4,800 credits Building near third area with collectors
Horizon Med Kit - Medi-gel Building near large set of pods
Horizon Computer Hack 3,000 credits Upper floor of building with stasis colonists
Horizon Particle Beam - Collector Particle Beam Area after third collector battle
Horizon Power Cells - Heavy Weapon Ammo Building near where Harbinger first appears
Horizon Weapons Locker - Choose Loadout Inside large building
Horizon Wall Safe - 6,000 credits Inside large building
Horizon Med Kit - Medi-gel Inside large building
Horizon Refined Platinum - 2,000 platinum Inside large building
Horizon Power Cells - Heavy Weapon Ammo Left path through large complex
Horizon Dead Collector - Biotic Damage Right path through large complex
Horizon Computer - 3,000 credits Second level of large complex
Horizon Power Cells - Heavy Weapon Ammo Near building in transmitter area
Horizon Med Kit - Medi-gel Near building in transmitter area
Captain Bailey Tali