The leaked Halo event in Forza Horizon 4 is definitely real, and it's a hilarious race through the series' tropes.
In a behind-closed-doors presentation at Gamescom today, creative director Ralph Fulton showed off the in-game event.
Following the template of the Horizon series' over-the-top Showcase events, and riffing on the original Halo game, the event will appear on your map and, once activated, kicks off a cutscene that sees Master Chief walk up to a Warthog, stare down a Pelican and then appear on a Northumberland beach overlooked by Bamburgh Castle. This is particularly weird if you're British.
The mission is framed as a training simulation, and has a set of Warthogs racing to keep up with a Pelican, avoiding swooping Banshees, and trying to get to the finish line before the Halo ring you're on (and yes, the skybox has been changed to reflect that detail) apparently collapses.
A fully-voiced Cortana narrates throughout the mission, commenting on what's going on around you, and mentioning the "ancient structures" (old British houses) you pass on the way. It's a surprisingly fully-fledged little addition - the fields are scattered with Covenant fortifications, and even the race barriers are made of hard light - and genuinely funny in its execution.
It's not clear if there's more than one Halo event in the game, but Fulton made clear that this one was replayable as many times as you want to play it.