Crazy Crossover AU Idea: Xi is the actual murderer of the nine men that Sebastian Solace was falsely accused of killing.
And the Crimson Knight of Epsilon literally used some insane methods to do so.
So basically, the basic plot of the AU revolves around the nine men in question (plus Sebastian himself) discovering something anomalous that they shouldn't have, alerting Epsilon and ordering Xi to murder those nine men with stealthy and outright anomalous methods, making the whole bloodbath as something Sebastian, now the sole survivor of the whole tragedy, did for nefarious purposes... and the rest is history.
Imagine Sebastian finds out the whole truth about this and becomes really hateful not only towards Xi, but also really hateful towards whoever-the-fuck Xi works for (cough cough Epsilon and his forces cough cough).
This is really a basic-ish summary of the AU that I gave and I have had this idea in my head for a while now. Go wild with it if you can.