The Halo series has finally returned to PC, although probably not in the way most PC gamers would hope. 343 Industries, developer of the most recent Halo games for Xbox, has announced Halo Online as a free-to-play online multiplayer game, but unfortunately it's exclusively for the Russian market.
Halo Online will be developed in conjunction with Saber Interactive and Innova Systems, and is a heavily modified version of Halo 3 designed specifically for low-end gaming PCs. There are no plans to bring Halo Online to the Xbox 360 or the Xbox One, and 343 Industries isn't saying whether the game will be seen outside of Russia.
The game will be set on a "secret UNSC space installation called Anvil", where Spartan soldiers train in "war exercises" to improve their battle skills and test out new technology. Halo Online will feature an in-game currency, almost certainly linked to real world money as most free-to-play games tend to be.
343 Industries says that Halo Online is "a learning opportunity for us as we explore ways to welcome new fans to the Halo universe". It will launch in the spring as a closed beta for Russians, though there's currently no word on when the game will launch as a final version.