Microsoft have not announced that Halo 5 is coming to PC. However, they have announced that the Xbox One shooter's sandbox mode Forge is coming as a free standalone to PC via Windows 10, under the rubbish name Forge – Halo 5: Guardians Edition. It's due "later this year". As I understand it (and I may be wrong!), Forge is a bit like a simpler Garry's Mod. Players can build levels collaboratively online, but also create and play fancier things like minigames. Good? Cool? This might also bring us closer to seeing proper Halo games on PC again.
So! Forge isn't a from-scratch level editor, more a sandbox where players can drop objects and drag them around and combine them to build things, then fiddle with lighting and weather and all that. It also has a little logic to tweak, so creative folks can make minigames and things. Multiple players can join in at once to build and test (play!) creations, so we may not have Halo 5 but we will have a game where we can kinda play a bit of Halo 5. Only we'll be allowed to use a mouse and keyboard (to build, at least, and presumably to play as well?). It'll let folks save Forge creations for people on Xbox One to play too.
I am certainly in favour of big fun free sandboxes. Here, for an example, is an Xbone Forge minigame where one player controls a giant death robot and everyone else tries to destroy it:
This does also mean that Halo 5 is basically on PC now. Microsoft had teased that a PC release was kinda sorta a possibility maybe, and this certainly makes it seem a lot more likely. Though as the weird, cut-down, singleplayer-only version of Forza 6 shows, part of a game running on PC doesn't mean we'll get the full thing. It seems Microsoft are still testing the waters.
2004's Halo 2 was the last proper Halo game released on PC (much later, in 2007), though Microsoft did chuck the unremarkable top-down shooter spin-off Spartan Assault our way in 2013.
This tweet from 343 Industries creative head Josh Holmes teases more Halo-y news to come, though it's vague enough to be about anything and hey, I tease things on Twitter all the time yet no one calls me out when I fail to chug a pint of self-tapping screws.https://twitter.com/JoshingtonState/status/733331895606378496
Somewhat less exciting than death robots, but perhaps more practical, is a trailer giving a bit of an overview of Halo 5's Forge on Xbox One: