Mephisto, Baal, and Diablo
In the cosmology of the Diablo games Diablo IV Items, the Prime Evil are demon lords of peerless power and malevolence. There are 3 total Prime Evils Mephisto, the Lord of Hatred, Baal, Lord of Destruction, and Diablo, the Lord of Terror and namesake of the dark fantasy action RPG franchise. In their prime, these Demon Lords led the forces of hell against the forces of heaven and sought to corrupt and turn humanity to their side when they learned about the existence of Sanctuary. In time, though, the Prime Evils were betrayed by the lesser demon lords who followed them, their bodies destroyed and their spirits trapped within artifacts called Soulstones.
What The Soulstones Do
Once cast into the human world of Sanctuary, the Soulstone prisons of Mephisto, Baal, and Diablo became artifacts of doom. Proximity to the Soulstones let these Prime Evils whisper into the minds of humans, corrupting or driving them mad and vulnerable to possession. Indeed, the plot of the first three Diablo games (particularly Diablo 2 and Diablo 2: Resurrected) revolves around the Prime Evils seeking to escape from their Soulstones and possess human hosts to regain their strength. Though none of the Prime Evils are fought in Diablo 4, the cunning Mephisto does show up to (for self-centered reasons) aid the player character in defeating his own daughter, Lilith.
Origins Of The Horadrim
In a world full of corrupt religious institutions, savage cults, and crumbling kingdoms, the reclusive order known as the Horadrim is the most reliable and stalwart force for good in Sanctuary, though individual members of the Horadrim have suffered greatly for this. In the backstory of the Diablo games, the original Horadrim was an alliance of mages and wizards, assembled by the benevolent Archangel Tyrael and charged with the task of defending Sanctuary from the schemes of the Prime Evils. It was these early Horadrim who fought the spirits of the Prime Evils, trapped them in Soulstones cheap Diablo 4 Items, and hid said Soulstones away from the world.