Microsoft will be running a promotion that will allow Xbox Live silver members to play Halo 3 online for free until Monday morning, in celebration of the release of the recent Mythic Map pack. The offer, which ends on Monday, April 13, at 9AM PST, is only for Halo 3 — other games will still require Xbox Live Gold to be playable online.

Halo 3-heads better boot up their Xbox 360 because it was recently announced that the Mythic Map Pack has officially hit Xbox Live for all to download.

Clocking in at 328.15MB the Mythic Map Pack will net you 3 new map back including Assembly, Orbital, and the all new Forge oriented map, Sandbox for just 800 Microsoft Points.

The official description of the pack, thanks to Major Nelson, is as follows:

The long-awaited Mythic Map Pack drops into Xbox LIVE Marketplace. Investigate Orbital’s abandoned hallways – before it was evacuated, the Quito Space Tether was heralded as the “Civil Engineering Monument of the Millennium.” Witness Assembly, where Scarab hulls loom overhead, being decontaminated in preparation for the city-sized Lekgolo colonies that will breathe life into the massive metal husks. An excavation site surrounded by deadly Forerunner defenses, the mysteries of Sandbox are hidden high above, deep below, and right before your eyes. There are no refunds for this item. For more information, see www.xbox.com/live/accounts.

And if you’re wondering why some people got the chance to play this before its Xbox Live street date, it’s because these same maps were included with Halo Wars.

If you didn’t already know, the late Ensemble Studios’ Halo Wars has hit North American shelves everywhere.

Don’t know what Halo Wars is? Have you been living under a rock? Only kidding, Halo Wars is the first official real time strategy (think Command & Conquer) title based in Bungie’s hugely popular Halo universe. Better yet, if you got the Limited Edition version, you now have access to Halo 3‘s Mythic Map Pack.

Oh yeah, it’s an Xbox 360 exclusive, but did I really have to say that?

I don’t know about you guys, but ever since the original Halo hit the shelves I’ve been a huge fan so you can see why I’d get excited over the following media.

Bungie has recently released new screenshots taken from it’s forthcoming Mythic Map Pack. 

Halo 3‘s Mythic Map Pack, which includes 3 all new levels, is set to be bundled with Halo Wars with a downloadable version hitting Xbox Live some time after.

Head over to EuroGamer to check out the rest of the screenshots.