In Mage: The Awakening terminology, a Reaper is an Awakened who has developed magic that relies on soul theft and abuse, usually via Legacy. In many ways, a soul, even a Sleeper one, is an untapped font of extremely powerful magic of all ten Arcana, making its use an extremely juicy temptation for the power hungry and ruthless. Needless to say, Reapers are almost as despised as the Scelesti, and are hunted wherever found.
The most infamous Reaper Legacy are the Tremere liches, who ironically are also the reason why other Reapers are thankfully rare; Tremere have the ability to "absorb" a Reaper Legacy they have destroyed and devoured the members of, creating a House, a sort of sub-Legacy in the larger Embalmed. The Tremere view this as their sacred right, believing that eventually, the knowledge they gain from absorbing new Legacies will eventually allow them to summon the Final Watchtower into the world and Supernal Realms , creating a golden age of magic in their own image.
While the Tremere absorb souls to extend their lives, other Reapers have motives like using them for supernatural luck and good fortune (the Carnival Melancholy), or to augment their own intelligence (Cloud Infinite).