From Halopedia, the Halo wiki
Haggar was the codename for a canceled Halo Mega Bloks title that deviated from the series' typical design and gameplay. The game was to be originally released on the Xbox 360. The game was apparently cancelled in 2013.
Haggar was an exploratory project worked on and intended for the Xbox 360 platform, in development "several years" prior to 2017. The game was worked on in conjunction with MEGA Brands and n-Space with a focus on elements of action, exploration and user creativity. The project was one of many Halo games prototyped by 343 Industries an an ongoing basis. The game was primarily created by n-Space, a studio which ultimately closed in 2016, using Unreal Engine 3 and was considered internally to be a fully-AAA project. As such, the prototype was developed to a more complete status than typical game pitches. According to PtPOnline, the prototype's work represents around 10 months of development (not including initial pitching) and the game was one of numerous pitches to Microsoft Game Studios around the same timeframe. The game's cancellation is said to have come suddenly by Microsoft.
On January 7, 2017, gameplay of the demo from the game's September or November 2013 build was leaked online and uploaded to YouTube by user PtoPOnline. Two days later, 343 Industries studio head Bonnie Ross made a post on the Halo Waypoint forums confirming the game's existence and the legitimacy of the footage shown.
Haggar's gameplay can be most likened to that of the LEGO video games series, particularly those produced by Traveller's Tales starting with LEGO Star Wars. The game was played in a third-person view and appeared to magnetically pull the crosshair over enemies to make aiming easier. Like Halo the player would have shields and health, be able to pick up and use weapons and grenades, and interact with various objects and structures to progress. New features specifically created for this game involved collecting blocks from fallen enemies and building various structures including barriers and automated turrets. Unlike the core series, vehicles could be customized with different paint jobs, decals, weapons, and tools that would give the vehicle different utilities.
In the prototype revealed on YouTube, one campaign mission vertical slice can be found extremely polished and almost complete, with another playable though lacking polish. In the first mission, players play as a Mega Bloks representation of John-117, on a mission to defend a UNSC base tasked with overwatching a Forerunner temple. En route to the objective, the Pelican is shot down by Banshees, forcing the player to proceed on foot through the swamps while battling Covenant forces along the way. According to Bonnie Ross, the prototype did not progress far beyond the levels showcased in the demo.