These are just few notes for myself. If they are any good for you then great!
Where you can find Reaper's Fear
Acquired from: Act V, Heart of the Fortress, Malthael (Drop rate is currently lower than intended.)
Where to farm: Act V, Heart of the Fortress
Used for: Reaper's Wraps, Wrists, Health globes restore 25–30% of your primary resource, 5 random magic properties (as opposed to the 8 properties according to Battle.net)
Note: Malthael also drops the Recipe for Reaper's Wraps.
