Sacrifice of the Widow

This the first book in the Lady Penitent series by Lisa Smedman. Released in January 2007.

"Halisstra Melarn, convert to the cause of the goddess Eilistraee, was sent to the deepest depths of the Outer Planes to kill the demon goddess she once worshiped, but instead was transformed into a hideous creature bent to the vengeful will of her former mistress. For the Queen of the Demonweb Pits not only survived her Silence but evolved into something greater than she was before -- something that no longer needs to share the domain she calls her own.

Could it be that the War of the Spider Queen has just begun?"