Originally developed as a real-time strategy game for the Apple Macintosh platform, Halo: Combat Evolved went through several iterations before arriving at the console first person shooter for which it is recognized. When the developers, Bungie, were bought by Microsoft in 2001, the game was rapidly finished and became an Xbox launch title and platform exclusive. Released on November 15, 2001, the Xbox version of Halo: Combat Evolved is the first Halo video game. The games of the main trilogy are first-person shooters, with the player experiencing most action from the protagonist's perspective. Combat Evolved introduced many gameplay and plot themes common to the whole trilogy. Players battle various aliens on foot and in vehicles to complete objectives, while attempting to uncover the secrets of the eponymous Halo. One concept introduced in Halo: Combat Evolved, is limiting the number of weapons players could carry to two, forcing them to carefully select their preferred armament. Players fight with ranged and melee attacks, as well as a limited number of grenades. Bungie refers to the "weapons-grenades-melee" format as the "Golden Triangle of Halo", which has remained fundamentally unchanged throughout the trilogy. In Halo: Combat Evolved, the player's health is measured in both hit points and a continually recharging energy shield. A PC and Mac port was later developed by Gearbox Software, and released on September 30 and November 11, 2003 respectively. A stand-alone expansion, entitled Halo: Custom Edition, was released as a PC exclusive, and allowed players to create custom content for the game.
Its sequel, Halo 2 was released on the Xbox on November 9, 2004 and later for Windows Vista on May 17, 2007. For the first time, the game was released in two different editions: a standard edition with just the game disc and traditional Xbox packaging, and the Collector's Edition with a specially designed aluminum case, along with an additional bonus DVD, extra booklet, and slightly different user manual. Halo 2 introduced new game-play elements, Chief among them the ability to hold and fire two weapons simultaneously, known as "dual wielding". Unlike its predecessor, Halo 2 fully supports online multiplayer via Xbox Live. The game uses "matchmaking" to facilitate joining online matches by grouping players looking for certain types of games. This was a change from the more traditional "server list" approach which was used to find matches in online games at this time. Upon release, Halo 2 became the game played by the most people on the Xbox Live service that week; regaining this title every week for over two years — the longest streak any game has held the spot.
Halo 3 is the third and final game in the main Halo trilogy, ending the story arc begun in Halo: Combat Evolved. The game was released on the Xbox 360 on September 25, 2007. It adds to the series new vehicles, new weapons, and a class of items called equipment. The game also includes a limited map-editing tool known as the Forge, which allows players to insert game objects, such as weapons and crates, into existing multiplayer map geometry. Players can also save a recording of their game-play sessions, and view them as video, from any angle.