[4] After Diablo is defeated by the Nephalem (the player character), Tyrael recovers the Black Soulstone that contains the essence of all seven of the Great Evils.

Knowing it is too dangerous to leave in the hands of mortals or angels, he and six Horadrim take the Black Soulstone back to Sanctuary and attempt to seal it away where it can never be found - deep in the tomb of Rakkis, the first King of Westmarch, the kingdom established to the west of Khanduras.

Now calling himself the "Angel of Death", Malthael kills all but one of the Horadrim, incapacitates Tyrael in the process and steals the Black Soulstone.

Nahr encounters the Nephalem outside Westmarch City, which has been overrun by the Reapers, Malthael's army of enslaved spirits and renegade angels.

Lorath decides to accompany the hero, then learns that Adria sealed the tomb's entrance with a rock slide, so he suggests unlocking the guide-stones to open the correct passageway.

Travelling into the ancient ruins in the Blood Marsh outside Westmarch, the Nephalem confronts Adria, who manages to locate Malthael at the Pandemonium Fortress (last seen in Diablo II), built in the realm between Heaven and Hell to watch over the Worldstone.

Upon defeating the attackers, they are met by Imperius, Aspect of Valor and the commander of Heaven's armies, who reluctantly admits that Malthael must be stopped and leads the Nephalem into the Realm of Pandemonium.

Imperius directs the Nephalem to use an ancient battering ram to breach the fortress gates, using siege runes held by the demons trapped there.

Tyrael arrives just as the Nephalem prepares to activate the ram, revealing that he has discovered Malthael's plan; he intends to use the Black Soulstone to consume all demonic essence in Sanctuary, including that which makes up the bloodline of humanity, leading to its extinction.

After breaching the gates with four hits with the battering ram, Tyrael informs the Nephalem that they must become "one with death", as Malthael is, in order to defeat him.

Inside the fortress, the Nephalem encounters a figure from their past (dependent on their class) who directs them to unlock the soul prison kept in its depths.

The Nephalem channels the spirits from the prison and takes on an aspect of death themselves, before moving on to defeat the guardians that bar the way to Malthael's sanctum at the heart of the fortress.

The patch changed many game systems, including class skills, difficulty, and the number and quality of items acquired.

Previously bind-on-equip and tradeable pieces of equipment were made account-bound instead, with a limited amount of time to allow trading to other characters who were present in game when the item was dropped.

In addition to the campaign, the expansion adds an "Adventure Mode", where players are free to roam the entire world and take on random Bounties and dungeons (called "Nephalem Rifts") at their leisure.

New objectives (including Bounties and Nephalem Rifts), areas and NPCs are also available to the players, and these features can only be found in Adventure Mode.

Until patch 2.7.3, the mode was unlockable (and made available for all seasonal and non-seasonal characters on the player's account) only after completing the entire campaign (including Act V) once.

Upon completion of Bounties, players will be rewarded with certain amount of gold, loots as well as crystalline blood shards, which can be traded for powerful items.

In Adventure Mode, players will speak to the ghost of Zoltun Kulle in town, at which point he will direct them to the Ruins of Sescheron in Act III to locate the cube.

In an Action RPG game based around RNG “random number generator” gear drops, you need as much inventory space as possible.

It will also allow for console gamers to earn the same season exclusive award previously only available to PC Diablo player base.

[22] New zones were also added to certain maps in Adventure Mode: The Shrouded Moors and the Temple of the Firstborn near Caldeum (Act II), centered around a strange "blood cult", and the Realms of Fate in the High Heavens (Act IV), consequences of the battle between the Nephalem (player characters) and Diablo in the campaign story in Diablo III.

Additional music was composed by Neal Acree, Joseph Lawrence, Russel Brower, Glenn Stafford, and Jason Hayes.

Diablo III: Reaper of Souls received favorable reviews upon release, with critics praising its iteration upon the base game.

[25] The expansion was lauded by many critics as a large improvement on most fronts from Diablo III and considered the changes made in it as responsible for the reinvigorated player interest in the franchise, giving the game greater replayability and long-term appeal.

IGN's Vince Ingenito praised the expansion's darker atmosphere, diverse gameplay, and overhauled loot system, stating "it’s more sinister in tone, more rewarding to play, and more maddeningly addictive than it’s ever been.

"[26] Polygon's Arthur Gies came to a similar conclusion, writing "Reaper of Souls reinvigorates an already great game".