From Halopedia, the Halo wiki
Halogen was a complete conversion mod of Command and Conquer: Generals. Developed by Slipstream Productions, the mod was an attempt to create a real-time strategy version of Halo but was subsequently cancelled following a cease and desist order obtained by Microsoft.
The mod was developed over three years prior to the cancellation. Powered by CnC: Generals's SAGE, the mod featured the traditional assets such as base-building utilities, resource gathering, and unit-management system.
Cease and desist order
Microsoft took notice of the mod's development and obtained a cease and desist order against the production group. On September 9th, 2006, the production group made an official announcement confirming the cancellation of development.
Following the closing of Halogen, the production group recycled the developed assets for a different project titled "Asylum" (later retitled to "Signal").
Halogen and Halo Wars
Halogen was cancelled in September of 2006, following a request by Bungie's community manager to the Halogen team sent on September 8. Although Microsoft had wanted to send an official cease and desist, no legal action was taken due to Bungie stepping in and asking Microsoft not to use legal action. The team had heard rumours as early as mid-August that they may need to halt production, and rumours circulated at the time that Bungie was preparing to announce their own RTS Halo game.
On September 27, 2006, the official Halo RTS Halo Wars was announced by Microsoft at the X06 media briefing event, showcasing a new animated trailer for the property, all-but-confirming that the cancellation of HaloGen was due to the imminent announcement of an official Microsoft competitor.