A Reaper is a harbinger of death for local Populations, and for any units in the province that the Reaper doesn't like; essentially a less focused Population Killer. There are two types: dedicated Reapers, and fellows burdened with the trait as a check on their power.

Whenever a Reaper is in the province, they Disease as many units in the province as their trait value indicates. For example, a Harvester of Sorrows has a Reaper value of 10%, so they Disease around 10% of the units in the province. As for who is selected, that is random. Just to be clear, a Reaper will not Disease troops and commanders on their own side.

A Reaper will devastate whatever province they're in, however, as though they're a blight on the land; in fact, the internal name of the trait is "Leper". Each turn, they kill around half as many taxpayers in the province as their trait value indicates; for example, a Harvester of Sorrows will kill 5% of the Population of whatever province they're in, each turn. They also cause Unrest; the amount is an open-ended die result, with the maximum being the total Reaper value in the province (times 100), so a Harvester of Sorrows would also apply around 6 Unrest each turn.

The effects of multiple Reapers in the same province are added together. Thus, if there are three Harvesters of Sorrows in the province, it's the same as though there is one unit in the province with a Reaper value of 30%. This is not ideal for Unrest-generation, but it's preferable for unit-Diseasing and Population-killing.

A Reaper cannot affect those inside a castle during a Siege; apparently, iron gates hold out the spectre of death as well. On the other hand, a Reaper being Besieged will affect the Besiegers just fine.

A Plague Doctor is a professional who has learned how to control the Reaper trait, to an extent. The fellow may be ordered to Spread Plague, in which case they'll start behaving like a Reaper; except, in the case of a Plague Doctor, they will Disease friendly units as well.