From the introduction:
Welcome to Tristram a small town in the region of Khanduras who’s only real claim to fame is the ancient cathedral on the northern edge of town. This land is in desperate need of a hero thanks to the army of demons and undead living under the cathedral. In recent times a strange alien nest has grown to the southeast of town across one of the streams that borders the town. This strange alien growth makes the townsfolk more than a little uneasy and yet there is little they can do on their own. To make matters even worse for the townsfolk, an ancient crypt near the cathedral has started glowing at night. You’ve been tapped to be a hero or at least the jumper that rids the world of Diablo however temporarily that might be.