The Spartan Armor
The Spartan armor is also called Mjolnir armor, Halo armor, Master Chief armor or Hal's armor. It is a suit of armor modeled after the suits worn by the soldiers in the Halo game series, although it has a slightly medieval look that's more in keeping with the Fable 2 universe. The armor is one of the bonus items available to players who bought the Limited Edition version of Fable 2. The in-game description explains that Hal from the Halo universe stepped through a portal into the universe of Fable, eventually dying and leaving his armor for the player to find.
Other Halo Items
As well as the green Spartan armor, the Limited Edition content includes the energy sword from Halo; in Halo, this is a weapon made from energy but in Fable II it behaves like any other sword. You can also obtain a Halo-style rifle to complete your outfit. The rifle is on Knothole Island so you'll need to own the Knothole Island expansion to get it.
Finding the Items
If you have the Limited Edition version of the game and have downloaded the "Otherworldly Content," you can find the Spartan armor the first time you level your character up. Once your character reaches adulthood and you complete your first quest, travel to the Cullis Gate and look for the Guild Cave nearby. Inside the Guild Cave you'll find a chest containing Hal's Spartan armor.
Trading for the Armor
If you don't own the Limited Edition version of the game you can't find the Spartan armor hidden in the Guild Cave. Instead, you must trade for it with another player; this is called haggling. You'll need to talk to other online players until you find someone who is willing to part with a suit of Spartan armor. Go to the "online options" in Fable II and enable the "Orb" option to interact with other online players. You can also visit online Fable II gaming communities and ask members there if they are interested in making a trade.