The Order fights a losing battle. There is a traitor among them who is fixated on killing them off. One by one they fall, like chessmen being brushed aside. The lone Slytherin is naturally suspected, even by the ones she thought she could trust. But even as war looms upon their doorsteps, happiness can still be found in the cottage Vivian now calls home...until, of course, someone from her past returns with information that could change the tide of battle, tragedy strikes, and the world that she has built for herself comes crashing down within a matter of days. A new purpose rises from the ashes, and with it comes a new Vivian Blair, who must learn to live without the ones she has come to love. Sirius/OC Season 2 of Vivicendium

Vivian Blair is the summation of everything Sirius hates: a prefect, a pureblood, and a Slytherin. It doesn't matter that she makes a half decent Seeker, has an unnatural obsession for reading muggle books, or even that Lily thinks she's a reasonable human being. Vivian is just a bitchy Slytherin who likes to call him a blood traitor and give him detentions. Fortunately, he has an idea to get back at her for everything she's ever done to him. He should know better than to toy with fate, for the more love poems he writes to Vivian Blair, the less of a prank they become, and the more fate seems intent on getting some revenge of its own. Sirius/OC