The Alliance Slayer

You get this quest automatically shortly after you become a Horde Assassin (I'm still not sure if you need to have a kill after becoming one or if it's time-based). I tried completing it in a raid, but it's NOT possible. Only possible if you are solo or in a group of 2-5. Levels of Alliance players we killed were between 110 and 120, so you don't have to only kill max level ones. Killing the same target multiple times also counts.

If you die you have to repeat the whole process all over again, UNLESS a friend of yours combat resurrects you and shares the quest with you again.

PS.The fastest way to do this quest is to create a raid and then hunt down players until everyone becomes an Assassin. Then, split into regular groups of 5 and roam together. Make sure each group has a healer. I tried to do it solo, and with a friend, but in the end we were taking too much time hunting for people that they organized into a mini raid of 10-15 and hunted us down instead.

Good luck!

EDIT: For anyone wondering, this is NOT account-wide. You will need to do this on a per character basis unfortunately.

Logging out also automatically fails the quest.

The quest will be offered to you when you become a bounty target.
It will fail if you die.
You can reset your [spell=264314] and bounty status, without failing this quest, by leaving the zone for a couple of seconds.