From Diablo Wiki
The Harvest event is triggered when you encounter Lord Dewhurst and his guards in the Westmarch Commons in Act V. This event is divided in two steps. First you need to kill the Reapers (Revenant Archers, Revenant Soldiers, and Revenant Shield Guards) that attack Lord Dewhurst's guards. Once this is done, you will be asked to escort the refugees to a safe place but on the way to safety you are ambushed by another wave of Reapers composed of Punishers, Exorcists, Death Maidens and the Summoned Archers they generate. The second wave can be pretty nasty. Caution is required, especially at higher difficulty levels.
Dialogs during The Harvest
Lord Dewhurst: "My personal guard is the best that money can buy! They will save us"
After the first wave of Reapers is disposed of.
Lord Dewhurst: "I thank you for assisting my guards. You are a true hero. Now, let us depart before more monsters arrive!"
In front of the gate to safety.
Lord Dewhurst: "Wait - what's this?!"
Just before the reapers (Punishers, Exorcists, Death Maidens) attack.
Death Maiden: "No one escapes the harvest of souls, you fool!"