Well Therion already answered your question and linked you a build.
Personally for Muti in BGs only, I'd go with something more like
this.
While MoD and Dirty Tricks are "must haves" in Arena, they aren't all that useful in BGs compared to what you're giving up in Assassination to get them. You can pick up a lot more burst damage by going 51 points in Assassination. Also, Blood Splatter is not likely going to be that useful in PvP.
By going 51 points in Assassination, you're able to pick up 9% more crit from 3/3 Master Poisoner and 3/3 Turn the Tables, (which should be active pretty much the entire time. Also, this build would use KS as its initial finisher, for the 9% extra damage. Just make sure you get a 3 stack of HfB before breaking stealth, and this build has some pretty crazy burst.