It's introduction in Halo 3 was massively welcomed by the community, so not having Forge available at launch was a small bone of contention for many fans. But in their latest major patch, 343 Industries today confirmed that the content creator mode is back in the game -- along with a basic File Browser to boot.
Forge is part of the Cartographer's Update, a free patch available now for all Halo 5 owners. But the mode has gotten some significant changes since its last implementation, according to the post on 343's website.
You'll have access to over 1700 objects, real light sources, fixtures, primary, secondary and tertiary colours, the ability to select and edit up to 64 objects at once, as well as groups that can created with a single button press.
It's actually quite complex, so much so that people are already actively lobbying for an in-depth Forge tutorial. The Forge controls aren't customisable either, and it might take you some time to get used to all the inputs.
The patch also adds a new asymmetrical map designed for Strongholds and Slayer, called Overgrowth (reminds me of the Call of Duty 4 map). Antifreeze and Entombed have been added for big team battle maps, while there are now social playlists available for those who want a less intense experience.
343 has also chosen to kick off the first season of their ranked Arena play, with the January 2016 season starting as of right now. Other smaller, but noticeable changes, include the reduction of the FFA playlist to 6 players from 8, the addition of Holiday Doubles to the ranked playlist, inclusion of a custom game option for Machinima controls, more REQs, the ability to play and pause film while spectating while the UI is minimised, more improvements to matchmaking ping times and a boost to "gameplay quality under network conditions with high packet loss".