Take heed and bear witness to the truths that lie herein, for they are the last legacy of the Horadrim. Nearly three hundred years ago, it came to be known that the Three Prime Evils of the Burning Hells had mysteriously come to our world. The Three Brothers ravaged the lands of the east for decades, while humanity was left trembling in their wake. Our Order - the Horadrim - was founded by a group of secretive magi to hunt down and capture the Three Evils once and for all.
The original Horadrim captured two of the Three within powerful artifacts known as Soulstones and buried them deep beneath the desolate eastern sands. The Third Evil escaped capture and fled to the west with many of the Horadrim in pursuit. The Third Evil - known as Diablo, the Lord of Terror - was eventually captured, his essence set in a Soulstone and buried within this Labyrinth.
Be warned that the soulstone must be kept from discovery by those not of the faith. If Diablo were to be released, he would seek a body that is easily controlled as he would be very weak - perhaps that of an old man or a child.