Six Hundred Years Ago?
In the Underdark, a drider who called himself "The Forsaken" crafted the rod known as Dark Fate, imbuing it with all his hatred of drow. When it was complete, he performed an unholy ritual and sealed himself into the rod, imbuing it with intelligence as well. Before his death, the Forsaken arranged for the rod to be delivered as a gift to a cleric named Alantria, the leader of one of the more powerful drow houses.
The power and ego of the rod overwhelmed Alantria; it utterly controlled her, started a civil war among the drow, and caused countless deaths. In what would be the last battle of the 'Tween Wars, Alantria was slain, and the rod passed into history, likely buried with the fallen champion in her ornate crypt within the war-ravaged drow city.
The Last Few Centuries?
Drow moved away from this area of the Underdark. Driders moved into the abandoned crypts.
A comet known as the Harbinger appeared in the night sky. As many view such celestial events as important signs, the comet's appearance sparked interest in researching its history. Scholars among the surface dwellers and the drow, endeavoring to discern its meaning, unearthed a prophecy written about the time of its last appearance. The prophecy seems to concern the comet's return (see below).
A group known as "the Holy" arrived in the town of Gryth, asked questions about the Underdark, and left the next day. (They didn't tell locals of their mission: to discover the rod called Dark Fate on behalf of their church.) Things went poorly for the group, though, as they were attacked by a number of Colossal monstrous spiders and driders. The spiders' poison affected more than one party member, including their guide -- a bard named Morning Glory -- and the Holy's leader called for a retreat. Unable to carry off all the wounded, one group member took a hair from Morning Glory's dead body and left it, planning to resurrect her once they got back to town.
However, the bard was not dead, only dying. The driders stabilized her and went to work transforming her into one of them.
Two Days Ago?
The transformation complete, Morning Glory Blade is now an angry drider, filled with loathing for the mortal world -- especially her former friends, the members of the Holy. She plots to find Dark Fate on her own and use it to wreak her vengeance.
The Holy returned to Gryth and sought to resurrect their bard, but the spell failed.
The PCs arrive in Gryth. The drow wizards Alakara and Pharn arrive in the Northern Caverns of the crypts and begin their own search for the magical rod.