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Acts are a gameplay feature of Diablo II, III, and IV.

Diablo II[]

There are four Acts in Diablo II, with an extra Act in Diablo II: Lord of Destruction. The player begins in Act I and moves through each Act in order as each one is completed (always ending in fighting a Great Evil). This is because the hero is tracking down Diablo, who visits each of these places before the player does. Traveling from the Rogue Encampment to the Kurast Docks is traveling across Sanctuary from West to East. The Pandemonium Fortress is located in the Burning Hells, while Harrogath (from the expansion) brings the player back to Sanctuary into the Barbarian region where Baal's army is attacking.

Act Name of chapter Town Final Boss
Act I The Sightless Eye Rogue Encampment Andariel
Act II The Secret of the Vizjerei Lut Gholein Duriel
Act III The Infernal Gate Kurast Docks Mephisto
Act IV The Harrowing Pandemonium Fortress Diablo
Act V Lord of Destruction Harrogath Baal

Diablo III[]

The Act system returned in Diablo III.

Act Town Final Boss
Act I New Tristram The Butcher
Act II Hidden Camp Belial
Act III Bastion's Keep Stronghold Azmodan
Act IV Bastion's Keep Stronghold Diablo
Act V The Survivors' Enclave Malthael

Diablo IV[]

At launch, Diablo IV had a prologue, six acts, and an epilogue. Unlike prior games, the first three acts can be completed in any order, and within acts themselves, objectives can be pursued in any order. After completing the first three acts, the story converges in the fourth act.[1]