In the days of old, the world was a stage for continual battles between Light and Darkness. People were constantly terrorized by Ahriman, the Wizard of Darkness. Then a young man appeared, who would change the world forever.
His name was Jamseed. Using three rings given to him by the Wizard of Light Jamseed fought Ahriman and sealed him away. However, Ahriman, stripped of his power, still did not give up.
He sent his servant, the Snake King Zahhark, to attack the kingdom of Shanadar founded by Jamseed. Jamseed was killed and the world reverted to an age of darkness. When 1000 years had passed, a warrior named Fallidoon defeated Zahhark and brought peace back to the world.
When the world had all but forgotten these events, an empire called Eflaat rose up... Shanadar once again had to stand up and fight for peace...
Defenders of Oasis's setting and plot draw heavily from Zoroastrianism, Mesopotamian mythology and The Book of One Thousand and One Nights, particularly the stories of Sindbad the Sailor, Aladdin, and Ali Baba and the Forty Thieves.
Source: Wikipedia, "Defenders of Oasis", available under the CC-BY-SA License.