Thanks to this very detailed synopsis from SpoilerTV.com (after the jump), we can gauge the impact of the White Witch's arrival. Obviously there are major spoilers, so if you're not up to date on this season, stop here.
In the aftermath of the slaughter of her people, Marie Laveau seeks a truce with Fiona and the Coven... but her dark master, Papa Legba, demands his annual due. The next morning, Cordelia is horrified when she finally learns that Hank is a Witch Hunter... and Marie confesses hiring him to kill the Coven. Fiona is forgiving of Marie on the grounds that she was doing what she had to do, but is furious with Cordelia for having married Hank and letting him turn her against her own mother. They learn that Hank's father, Harrison, was his primary employer, and they cast a spell to make his corporation begin losing money, the better to weaken him. Later, Fiona attempts to out Misty as the next Supreme. When Misty proves uninterested in the position, Fiona tempts her by inviting a White Witch... Stevie Nicks, Misty's favorite singer. All the girls are enraptured by Stevie, except Madison, who is upset that Misty is the new Supreme-apparent, and, in her jealousy, goes to extreme lengths. Meanwhile, as Fiona becomes ever more ill, she asks Marie for the secret of her eternal youth, and Marie tells her the terrible price. As Fiona brings the Axeman a proposition and learns a startling truth about herself, Nan, wielding her growing powers, confronts Joan about Luke's death. And later, Marie and Fiona make a terrible deal – a deal involving a most unexpected betrayal.