Harvesters are those that abduct nulls to drain the energy from Nulls—which kills them—to make Null bombs. Harvesters are mostly vampires but some Mages are also harvesters.
Around the year 900, a mage figured out how to siphon away our energy, and thereby our lives, to make Null Bombs ombs capable of bringing all magic in an area to a standstill. How far and how long the effect extends depends on the strength of the null being sacrificed—the younger and more powerful they are, the more energy they have to give. After the process was discovered, it became fashionable to hunt us, especially in the vampire community, although some mages did it, too. Null Harvesting, as it's politely called, was outlawed shortly after the practice began, but the law had less to do with stopping the hunts than the meager quality of the nulls remaining by the Renaissance. Harvesters ran themselves out of business by being too good at their job, not that there weren't a few enterprising types still trying.