The Reaper Armor set is unlocked by completing Rebel Missions to gain Rebel currency and increase your Infamy. The Armor pieces are unlocked as follows:
Reaper's Pants is available to purchase for 400 currency at Infamy level 2
Reaper's Helm is available to purchase for 450 currency at Infamy level 3
Reaper's Cloak is available to purchase for 450 currency at Infamy level 3
Reaper's Bracers automatically unlock upon unlocking Infamy level 3 and starting your next Rebel Mission
Reaper's Torso is available to purchase for 750 currency at Infamy Level 4 (Max Infamy)
There are four Sycthe's you can obtain in this DLC:
Scythe of Revolt is unlocked as part of the "Majesty in the Dark" main quest.
The Bloodied Scythe can be found in a wealth chest in Amienois.
The Scythe of Tribulation in a gear chest in Paris
Wretched Scythe in a gear chest in Murmiliacum (Amienois)
Now, equip all of your Reaper Armor pieces and one of your Scythes. You can keep your bow equipped, but should remove any second hand weapon to avoid any issues. Then just kill any enemy with the Scythe and all the armor pieces equipped.
See the video below for more information.
Timestamps:
0:00 - Full Reaper Armor Set
1:17 - Scythe of Revolt
1:46 - Bloodied Scythe