Kill 12 Rattlecage Skeletons, and then return to Shadow Priest Sarvis in Deathknell when you are done.
You've shown your potential to the Forsaken under normal circumstances [player] - now let's see you under pressure.
The Rattlecage skeletons, more mindless minions of the Lich King, are a tougher foe than the zombies you have faced thus far. Again, thin their numbers and prove yourself to the Forsaken. Do not tarry - when you are done, speak to me again.