The Ghost hunter backpack is part of the Ghost hunter equipment. It could be bought with the ghost hunter body for 8000 reward points during the 2014 Halloween event. It comes in three levels: tier 1 is level 20, tier 2 is level 50, and tier 3 is level 70.
Ghost Hunter equipment has a set bonus that increases experience earned and damage done against ghosts and related creatures. This bonus is 3% with one piece, 6% with two pieces, and 10% with three or more pieces of the set.
Since the Halloween event ended, it can now be obtained as a rare drop from ghost type monsters, such as aberrant spectres.
This armor can be stored in the armor case of the POH. Make sure all four pieces are the same level by wearing them all because they won't store if different levels.