Invasion

Invasion is a gametype in Halo: Reach. An alternate mode of the gametype is a variant called Invasion Slayer, which is a six-versus-six Spartan-vs-Elites slayer variant.

Overview
Invasion is a team-based gametype and can be described as a large vehicular-based firefight. The game begins on Phase 1, where only one portion of the map and no vehicles are available. As the game continues, more Phases unlock meaning there will be more objectives to be completed, vehicles will become available, and more Loadouts will become available as well. Boneyard's version of Invasion features the Elites on the offensive, trying to obtain a UNSC data core with valuable information on it, while the Spartans are tasked with defending it.

Invasion Slayer is a six-versus-six Spartan -vs-Elites slayer variant where players will eliminate the opposing team in order to score points; obtaining a total of 100 kills will end the game. Similar to the default Invasion gametype, the slayer gametype will be regulated through Phases which can be unlocked by either acquiring a specific amount of kills or surpassing a specific time.

Players will not be able to choose which multiplayer model/teams they want to play in Matchmaking; the system will automatically assign the player to play as either a SPARTAN or an Elite.

Differences between Invasion & "Team" games
In addition to the remarks above a typical Invasion (non slayer) game can also be described as a mix of on sided territories & capture the flag. Accomplishing each objective results in more loadouts being available while failure to accomplish the objective, in the time allotted, results in either the other team going on the offensive or the game ending. A distinction that players may notice upon their first death is that they may opt to spawn in various locations, including "on" other teammates, similar to the co-op campaign game, this means that death may occasionally be useful.

A "useful death" may put a player in a position to better accomplish his/her teams goals by acquiring a vehicle before the opposing team can get it, automatically being put into the turret of a manned vehicle. Alternatively, if the player you are "spawning on" is in a fully manned Falcon or Banshee, the player will often fall to their death unless the jet pack is used. It is note worthy to mention that as the spawning only occurs outside of "combat" (not being engaged with an enemy or significantly close to one) Players who note the "spawn update" (an indicator with triple arrows alerting the player that someone wishes to "warp" to their location) are encouraged to "disengage" or back off their opponents to put their team in a more powerful position.

Use of teamwork is strongly encouraged as even a small team of two or three can dramatically affect the results of an invasion game through mutual spawning and taking full advantage of the multi-player equipment (especially the falcon on the spire map. The Utility Helicopter often being used as a "ferry" to allow the spartans to leave the spire without jet packs or jumping to their death. Further it allows the Spartans the ability to provide close air support between and around the spire and the power core drop off point). A final point on team work is that in slayer vehicles should be used to accomplish the objective whether it be racking up kills, holding back while your entire team respawns on you (thus allowing an en-mass attack on a weakly defended position), or "taking one for the team" by pulling enemies away from a territory or the power core so that someone else can go for the win, booting teammates for betrayal is highly discouraged due to players using split screened XBoxes (thus your team losing 1/3rd of its players) and due to the small number of players starting out with (remember, even a Noob can draw fire).

Difference between Invasion and Invasion Slayer
In invasion there is no "offense" or "defense" as one is accustomed to in traditional slayer. Players do however have a "base" which is best described as a set of points they can spawn into. Naturally an opposing team trying to occupy these points will, often, eventually be annihilated due to attrition, thus discouraging spawn camping.

The use of the "territories" in Invasion Slayer are spawn points for more powerful weapons, such as the sniper rifle and rocket launcher or vehicles such as a warthog, scorpion or other vehicle. The type of weapon depends on which side "takes the territory", covenant if the elites do and UNSC if the Spartans do. The use of the equipment or vehicle is first come first serve, regardless of who (both individually and as a team) acquired the territory. This further encourages players to engage in combat in the middle of the map as "capturing a territory" next to an enemy spawn point will often result in the loss of the equipment created (and the player(s) death(s)) when the enemy team respawns often with more suitable weapons. There is no "power core" in Invasion slayer.

Phases
The default Invasion gametype is broken into three phases; successfully completing an objective in one phase will unlock the next phase. If the Spartans can hold out during any one phase, the game is over and they score a victory. As a result, UNSC Longswords will fly overhead and clear the zone with a tactical strike, rendering any and all data stored on the core completely destroyed. Should the Elites be in control of the current objective, the game will go into Sudden Death mode where respawning is disabled.

In the Invasion Slayer gametype, each team has to reach a certain amount of kills or surpass a certain time period in order to proceed towards the next phase. Similar to the default gametype, new loadouts, weapons and vehicles will be available to the players as they proceed towards the next phase.

Phase 1
Phase 1 requires the Elites (Attacking Team) to shut down one of the two generators positioned inside the Commonwealth. The process of shutting down the generators requires 20 seconds. Successfully shutting down one of the generators will disable the security systems that leads to Phase 2 and the match will be extended for an additional four minutes. The Spartans (Defending Team) will win if the Elites fail to shut down the generators in less than four minutes allotted in Phase 1.
 * Default

Phase 1 is the first phase of the Invasion Slayer gametype and ends once the scoring reaches 25 kills or the timing reaches four minutes. The next phase will be unlocked if any of these markers are surpassed.
 * Invasion Slayer

Phase 2
Phase 2 requires the Elites to control one of the two newly marked territories; one located inside the refinery and the other located beside the Spartans' vehicle bay. Similar to Phase 1, a total amount of 30 seconds is required to successfully shut down the security systems. During this phase Elites gain a Ghost and a Wraith for their use, and the Spartans get a Warthog. Successfully completing Phase 2 will leave the data core exposed for Phase 3 and add an additional four minutes to the match. Elites will keep a total of two loadouts but with more powerful weapons, in addition, they will also get a weapons cache with a plasma launcher and/or a focus rifle. Spartans will also get two loadouts as well.
 * Default

Phase 2 is the next phase of the Invasion Slayer gametype and ends once the scoring reaches 50 kills or the time limit reaches eight minutes. The next phase will be unlocked if any of these markers are surpassed.
 * Invasion Slayer

Phase 3
In Phase 3, the Elites must retrieve the data core and deliver it back to the Phantom hovering onto the scaffolding near the ridge. When carrying the data core, the carrier's movement will be hindered by the package, similar to that when carrying support weapons in Halo 3, and will be left defenseless and vulnerable to the Spartans. Elites will unlock a Banshee in this phase, and Spartans unlock a Scorpion Tank. The Elites win when they successfully deliver the data core to the Phantom, also ending the game. Elites will have four loadouts. Spartans will have four equally powerful choices.
 * Default

Phase 3 is the final phase of the Invasion Slayer gametype and ends once the scoring reaches 100 or the match's time limit runs out. Surpassing either of these markers will officially end the game. There are also several areas to capture, once captured reinforcements will be dropped to your base. The higher your score the better the reinforcement.
 * Invasion Slayer

Invasion: Spire
Spire was a map not included in the Halo: Reach Beta. It has reversed roles; Spartans are on the offense trying to capture a tower held by the Elites. Vehicles that are on this map include: Falcons, Banshees, Revenants, Warthogs, Ghosts, and Wraiths. There are stationary plasma turrets for use along with multiple snipers for use on the platform and around the battle field. Similar to the Boneyard invasion, there are 2 optional places to capture in both the first and second phases. The first 2 areas that need to be captured are around covenant comm stations. Once those are taken Spartans must push onward to the next two objectives which are at the bottom of the spire. Then once those are taken Spartans can move on to the top of the spire to take the Data Core to a Pelican on the beach. But if the data core doesn't make to the pelican in time the Elites defending the core will win.

Invasion: Breakpoint
Invasion: Breakpoint is set to be Spartans on Defense, Elites on Offense. The first phase consists of the Elites taking out 2 Objectives. Once they have been taken out, the 2nd Phase is an Assault Phase, which consists of the Elites taking their bomb to the base and plant it. The final phase consists of the Elites taking the Core from the base to the extraction point. In phase 2 the Elites get a Ghost and a Wraith, and in phase 3 they get a Banshee. It is unknown when it is, but eventually the Spartans get a Warthog, a Falcon, and a Rocket Hog.

Trivia

 * A glitch in this gametype during the multiplayer beta swapped each team's role, causing the Elites to defend the Core while the Spartans tried to steal it. This also swapped the Loadouts for both teams, making Elites gain access to Spartan loadouts and vice versa, but this glitch was fixed for the final game.
 * Even though the default setting for Invasion is 6 vs 6, you can increase the maximum players up to 8 vs 8 in custom games
 * The "Marksman" Loadout was removed from the first phase of this gametype during the multiplayer beta due to Spartan players able to get multiple kills on Elites with virtually no cover.
 * If a territory's point countdown is pushed below seven but the attackers are successfully killed before the territory is lost, the territory's points can be 'recharged' back up to a maximum of seven. This is most likely another seven reference.
 * In Invasion: Breakpoint, the 2nd phase will consist of Assault, which is the first time this will be in Matchmaking.