Update: Microsoft has clarified that Forge—Halo 5: Guardians Edition will not support matchmaking or any other Halo 5 multiplayer features, and so custom maps and games created with it will only be playable online with people in your Friends list. The software will also import maps created with the Xbox One version of Forge, which can then be edited and played in the same way.
“Forge—Halo 5: Guardians Edition on Windows 10 is a free publishing tool designed for map makers and creators to build and play their creations on Windows 10 and also publish these experiences to Xbox One,” a spokesperson said. “As we’ve shared previously, we do not have plans to launch Halo 5: Guardians on Windows 10. We are focused on building Halo Wars 2 for Windows 10 and Xbox One. Releasing Forge for Windows 10 for free will open up endless opportunities to the PC community to build and play Halo 5: Guardians multiplayer content, while also allowing them to share their creations with gamers on Xbox One as well.”
Microsoft has announced that Halo 5's map making tool Forge is coming to Windows 10 as Forge—Halo 5: Guardians Edition. It will feature a number of enhancements for the PC including keyboard and mouse support and resolutions up to 4K, and it will be completely free—but it will apparently not include Halo 5 itself.
“Since we released Forge in Halo 5 we’ve seen so many amazing new things come from it—from weird mini-games we could have never anticipated to meticulously crafted maps that are even approaching the visual quality of internally built maps,” Tom French of 343 Industries said. “The team has continued to wonder what we’d get if we could put Forge in the hands of as many people as possible, and making it free on Windows 10 literally gives the chance to anyone who wants to jump in and try their hands at building a cool new map for Halo 5.”
Maps made on PC will be publishable on Xbox One, though apparently we'll also be able to test them on PC. “Enlist the help of friends to help build, test, and play your Forge creations on Windows 10,” says the announcement. “Experiences built on Windows 10 can be published to and played on Xbox One, opening the doors for countless new experiences to be enjoyed by players all over the world.”
Making the editor available for Windows 10 is eminently reasonable—a mouse and keyboard are far more efficient editing tools than a controller, after all—but doing so while holding back the actual game comes off as almost a slight, like we're being given the free opportunity to create more content for Microsoft's 'real' gaming audience.
It's possible that Halo 5 could be recreated, in whole or in part, in Forge, but the announcement doesn't indicate exactly how we'll play Windows-created maps with other PC players, just that it's possible to test them on Windows 10.
Microsoft says more information about Forge—Halo 5: Guardians Edition will be revealed later this year, and that updates to Forge itself, including new environments, pieces, palettes, props, and advanced filed sharing capabilities, will be rolled out to the Xbox One over the coming months. I've reached out to Microsoft for clarification, and will update if I receive a reply.