The ghost being fed on requires you to kill certain mobs that contain its soul, you need to kill as many of them as possible before the timer runs out, the red timer bar will fill with a yellow tint as you do so. I haven’t been able to complete this one yet.

I think it requires the highlighted mobs to be close to the ghost when you kill them, but it feels like way too few of the special mobs spawn so you usually have no chance of keeping up with the timer.. I've only managed to finish it once, when playing with a friend.. When playing alone there's just too many regular mobs spawning, and as a ranged rogue I can't hug the ghost and tank everything. Though most of the time when playing with multiple friends they just ignore the mastery objective and kill everything as soon as it spawns :/