Halo Online has been announced today by 343 Industries as a free PC Halo multiplayer; however the game will only be available in Russia.
Halo Online will be a purely multiplayer game with no campaign mode and the developers have announced that there are no plans to port the game to console, or at least the Xbox One in particular. The game is set on a secret UNSC space stationed called “Anvil” where Spartan-IV soldiers train for battle and test brand new experimental technology.
Closed beta for the game is due to begin later this Spring in Russia though no date(s) has been announced yet. As expected the beta will be continuously modified to ensure the game runs smoothly and all features, including an in-game economy, are glitch free. There is also not talk of a definitive launch date for Halo Online so the game could be in beta for quite some time.
Although the game is currently planned to be only for Russia the developers did have a comment about the possibility of the game being transferred to other regions.
Right now our focus is on learning as much as we can from the closed beta period in Russia. Theoretically, any expansion outside of Russia would have to go through region-specific changes to address player expectations.
Their statement does suggest they may consider expanding to other regions also; if they did consider this it would be long after the release of the game in Russia.
Halo is a series typically though of as being a console only game so for some fans this news may be somewhat of a shock however the series was initially intended to be released on PC before it was kept as a console exclusive; ports have also been made in the past by Gearbox Software who ported Halo: Combat Evolved back in 2003.
You can see the full announcement for Halo Online here