Act V of Diablo III was added with the release of Reaper of Souls, Diablo III's first expansion. It begins in Westmarch and ends in the Pandemonium Fortress. Malthael and his Reapers serve as the main antagonists, while Adria returns as a secondary antagonist.
The first part of this act is when you have to stop the demonic rituals that the reapers are doing on the townsfolk to kill them. This part of the act ends with the boss fight against a Maiden of Flame.