The second of the three Prime Evil brothers. Baal is noted as the most straightforward of the three. Descriptions show Diablo's lieutenant, Astrogha, mocking Baal and his minion Gulag as dim-witted. The journals of the three brothers found in Reaper of Souls also reveal a personality that acts before thinking, unlike his brothers. However, he is only 'simple' by the standards of the Prime Evils; one must not overlook his intellect. Befitting a Prime Evil of the Burning Hells, he is more intelligent and cunning than mortals can fathom. His destructive and cruel nature is expressed most intensely among the three. The Realm of Destruction where Baal resides is a place of active volcanic and seismic activity, with the Hellforge at its center. It is one of the few places in the Burning Hells ever seen by human eyes, and the weapons forged there clearly reflect Baal's nature. It also houses the 'Anvil of Annihilation,' capable of crushing anything.

The youngest of the three Prime Evil brothers. Diablo is one of the seven demon lords who rule Hell, and specifically a Prime Evil, belonging to the powerful top three alongside his brothers Mephisto and Baal. Although the youngest, his combat prowess is said to be the most formidable, and he can wield power close to his original strength within Sanctuary. In short, he is the most powerful being among the seven demon lords and the strongest in the Burning Hells. Beyond mere combat strength, and contrary to his imposing appearance, he is extremely cunning and intelligent. Like a true demon, he excels at corrupting humans and manipulating them for his plans. Though he is the Lord of Terror, Diablo himself feels fear. What Diablo—who fears neither death nor defeat—dreads most is the very sensation of terror being turned upon himself.

The only female among the Great Evils. Andariel is a demon intoxicated by mental sadism. Often called Duriel's twin, she differs from him in that while Duriel enjoys physical agony, Andariel focuses on emotional anguish and believes in the purity of suffering. She revels in the sight of a victim's mind being completely overturned and left in tatters by emotional pain. Little is known of her realm, but it is a site of psychological torment where guilt, regret, and self-loathing assault the body and mind. It is said that victims in this realm become so consumed by guilt that they seek eternal physical pain and wander into Duriel's realm of their own accord. As a demon who relies on the suffering of others to live, she is the most socially-oriented and is said to loathe solitude. Furthermore, for reasons unknown, she hates fire despite being born in the Burning Hells.

Duriel, who seeks physical pain, and Andariel, who is interested in emotional anguish, are called twins due to their natures. It is said that when Duriel runs out of victims to torture, he finds pleasure in inflicting pain upon his own body. Although little is known about the Lord of Pain himself, his realm is the most widely recognized through antiquities and literature. This is likely because his realm resembles the torture chambers of Sanctuary, making it easy for writers and artists to understand. True to its simple nature, the Realm of Anguish is a cavernous region filled with sadistic and grotesque torture devices. Duriel is said to derive immense pleasure from torturing thousands of captured savage demons, making it a place that other denizens of Hell avoid as much as the Realm of Terror. Even his sister Andariel is said to only enjoy the discord of screams echoing from the outside.

Belial advocates the logic that reality is merely a matter of perception. His sole goal is the domination of reality; he does not lie simply to deceive, but to manipulate the very perception of what is real. The Realm of Lies he rules is a paradise of beautiful and fertile lands, but victims deceived by this illusion soon see Belial's true nature. Venturing deeper, the place that seemed like a peaceful sanctuary turns into a filthy burrow that strips away one's sense of reality, and the delicious-looking fruits turn out to be rotten and filled with wriggling maggots.

Some people find his rotund appearance cute, comparing him to a fat cat. Despite his somewhat dull appearance, he is a brilliant field commander recognized even within his main force, the Army of Sin. He has handed several defeats to the angels during the war with the High Heavens. Azmodan pursues the ultimate pleasure in all things. He loves all forms of vice and can see right through a target's desires. He delights in the failures of others, and his greatest joy and thrill come from shattering all morality. He was once nearly killed by Tyrael.