Grim Reaper is a Legendary Two-Handed Mighty Weapon for Barbarian in Diablo III, obtained via purchase of Reaper of Souls Collector's Edition (or Digital Deluxe edition) as part of the Arma Mortis pack. It can only be used for Transmogrification.
Legendary Two-Handed Mighty Weapon
"I realize now I’ve made a terrible mistake. The knowledge that the power of human souls might be extracted and used to empower the angelic Host would immediately escalate the Eternal Conflict. And if demons were to learn these methods, too... I daresay the war would end in our mutually assured destruction. All life would be extinguished. No one must ever know what I’ve discovered." — Author Unknown, Armis Mortem