What is Diablo III's fifth act called?

Diablo III's fifth act is called Act V.

Act V was added by the Reaper of Souls expansion, which was released in 2014. It takes place mainly in Westmarch and the surrounding regions, where the player confronts Malthael, the former Archangel of Wisdom who has become the Angel of Death.

The original Diablo III campaign contained four acts. Reaper of Souls extended the story with Act V, introduced the Crusader class, raised the level cap from 60 to 70, and added the Adventure Mode endgame structure. These additions made the expansion more than a short story chapter.

A common source of confusion is counting the base game's four acts and assuming the story ends there. The complete campaign after Reaper of Souls has five acts, with Act V serving as the expansion's main narrative section.