Age of Empires Online. The DLC allows players to start straight away at level 20, allowing them to immediately tech up.
The DLC includes the Immortals, which are effective against infantry at short range but can also shoot arrows from afar, as well as the Sparabaras, a shielded infantry unit capable of withstanding ranged weaponry. Another special unit are the War Wagons, impressive war machines able to destroy enemies with volleys of arrows.
Persians also come with more than 50 new pieces of gear and two new warehouses. Other new buildings include the Aid tent, which can be built by upgrading storehouses and will heal nearby units automatically.
The purchase price for the Persian civilization is offered at the special introductory price of 640 Microsoft Points, rising to 1280 MSP after the limited promotion ends. More info on the Persian civilization can be found at the official site.