Freyja's Strand was an ancient gold necklace that allowed its wearer to shift forms.
Acquiring the coveted artifact required doing the bidding of the comically portrayed ice demon Skrymir, who possessed the necklace yet was oblivious of the trinket's priceless powers.
In order to transform the world into a realm of darkness in which he will be the ruler, Scrymir instructed Spike and Drusilla to kill all the Slayers-in-Waiting, a group of 20 or so girls spanning the globe who are trained to assume the role of Vampire Slayer if and when the current slayer, Sophie Carstensen, should die. (Spike and Dru: Pretty Maids All in a Row)