The Dark Reaper was a super weapon, created by the Brotherhood of the Sith after the Great Sith War. It used the Force Harvester to drain living things of the Force, which killed them, and then turned the energy into high-powered Force energy beams.
During the Clone Wars, Count Dooku hired the Bounty Hunter Cydon Prax to help him control his forces when he was looking for the pieces of the Dark Reaper, which were scattered throughout the galaxy, so that he could use it to help win the war. Cydon Prax also replaced Jango Fett who was killed at the Battle of Geonosis.
Search for the Force HarvesterEdit
Count Dooku's search for the Dark Reaper led him to the waste planet Raxus Prime, where he was looking for the Force Harvester. He then ordered Cydon Prax to use the Force Harvester of a moon of Kashyyyk, half of the planet was destroyed in the blast of energy. Dooku then searched many outer rim planets and managed to reconstruct the Dark Reaper.
Dooku's use of the Dark ReaperEdit
Dooku then went to Thule to test the complete Dark Reaper on an ancient Sith city. It was at this point that Anakin Skywalker was on Rhen Var, at the tomb of Ulic Qel-Droma, and learned how to resist the Dark Reaper and the Force Harvester. Anakin then led an attack on the city and destroyed the Reaper, disobeying an order from Obi-Wan Kenobi to wait for reinforcements. The knowledge he took from Ulic's teaching's were, indirectly, his first step to the dark side of the Force.
Few people were able to resist the Dark Reaper's Force harvesting, among them was Ulic Qel-Droma, whose teaching's were recorded and later stored in Rhen Var's ruins, which were also Ulic's tombs. Anakin Skywalker later viewed these teachings after evacuating Clone Troopers from various outposts to the main Republic base in a Republic Gunship.