Belial

Belial, Lord of Lies.

Belial, one of the Lesser Evils, is confirmed to appear in Diablo III, in Act II.[1]

Belial in Diablo 3

The Belial had once been a protege of Mephisto, honing his powers of deception under the Lord of Hatred's guidance. Eventually, he would turn on his mentor and manipulated Azmodan into leading a revolt against the Prime Evils, though this may have likely been indirectly encouraged by Mephisto himself. With the armies of Azmodan and Belial unified, the Prime Evils were cast out of Hell, though at the expense of a full third of Hell's demons. Azmodan and Belial were left to vie for power over the Burning Hells, a conflict Belial ultimately lost. Years later, much like the Prime Evils, Belial would turn his attention to Sanctuary and corrupting its populace. His influence over the mortal realm mirrors Mephisto's own of twenty years prior, as his power over Caldeum, capital of Kehjistan, becomes absolute.

Act II will be "a little bit more of an intrigue and deception-focused story, because Belial is in there."

The Lord of Lies

All greater and lesser evils of the Burning Hells have their own specific title or aspect, and Belial's is the Lord of Lies. Along with Azmodan, Belial helped in a rebellion against the Prime Evils to seize control of Hell. From the Diablo I Manual:

In their ignorance, the Lesser Evils began to believe that the Three were afraid to continue the war with Heaven. Frustrated by the cessation of the war, Azmodan and Belial saw the situation as their chance to overthrow the Prime Evils and take control of Hell for themselves. The two demon lords made a pact with their minor brethren, assuring them that the wretched plague of humanity would not deter the ultimate victory of the sons of Hell. Azmodan and Belial devised a plan to end the stalemate, achieve victory in the Sin War and ultimately ride the bloody crest of the Great Conflict straight into the very arms of Armageddon. Thus, a great revolution was set into motion as all of Hell went to war against the Three Brothers...

The Brothers fought with all of the savagery of the Underworld, and to their credit, annihilated a third of Hell’s treacherous legions. In the end, however, they were overcome by the Horned Death led by the traitors Azmodan and Belial. The Prime Evils, weakened and bodiless, were banished to the mortal realm where Azmodan hoped that they would remain trapped forever. Azmodan believed that with the Three set loose upon humanity, the Angels would be forced to turn their focus upon on the mortal plane - thus leaving the Gates of Heaven abandoned and defenseless. Those few demons who still pledged allegiance to the Three Brothers fled the wrath of Azmodan and Belial, escaping to the realm of Man to seek out their lost Masters.

As the warfires died out upon the battlefields of Hell, Azmodan and Belial began to argue over which of them held the higher authority. The pact that they had made quickly fell to ashes as the two demon lords took up arms against each other. The legions of Hell that remained were polarized behind either warlord, launching themselves into a bloody civil war that has lasted to this day...

While it's important to note that the information in the Diablo I manual is "open to interpretation" (or retcon as it is often referred to), it is also relevant to the plot of Diablo III. Jay Wilson made this comment in a fansite interview[2]:

If the player thinks they outsmarted the forces of Hell and evil and they basically won, well, they are in for a big shock. Because what they are going to discover in Diablo 3 is that things have gone pretty exactly the way Hell wanted them to go all along.

This could refer to plots laid out by Azmodan and Belial, or to the Worldstone, or to both and more. It remains to be seen precisely what role the lesser evils will play in Diablo III.

Media

Walkthrough Guide for Belial | Solo Monk | Hell difficulty

Reference

Belial, Lord of Lies. Concept art shown at Blizzcon 2011

Concept art.
Concept art.
Concept art.

Boss and Champion Special Boss Modifiers
Uniques Modifiers	Strong CC (Limit 1): Knockback -- Nightmarish -- Vortex -- Jailer Defensive (Limit 1): Avenger(C) -- Health Link(C) -- Horde(B) -- Illusionist -- Missile Dampening(B) -- Shielding Aggressive (No Limit): Arcane Enchanted -- Desecrator -- Electrified -- Fast -- Fire Chains(C) -- Frozen Pulse* -- Frozen -- Molten -- Mortar -- Plagued -- Poison Enchanted* -- Reflects Damage -- Teleporter -- Waller (B) = Boss Only modifier. -- (C) = Champion Only modifier -- * = Reaper of Souls only
Removed Modifiers	Ballista -- Cold Aura -- Die Together -- Doppelganger -- Extra Health -- Invulnerable Minions(B) -- Life Link -- Magical -- Mirrored -- Multishot -- Mythical -- Powerful -- Stoneskin -- Vampiric
Key Quest Uniques and their Special Modifiers
Act One Uniques	Araneae - Web Spit	Leoric - Summon Skeleton, Teleport Strike, Triple Cleave		The Butcher - Smash, Grappling Hook, Spear, Frenzy Charge
Act Two Uniques	Maghda - Forcefield, Summoning Ritual, Punish, Moth Dust, Teleport.	Zoltan Kulle - Fiery Boulder, Time Bubble, Ceiling Collapse, Energy Twister,Vanish, Creepy Laugh		Belial - Phase One: Triple Fireball, Summon Vipers. Phase Two: Poison Meteors, Ground Pound, Lightning Strike, Lightning Breath.
Act Three Uniques	Ghom - Acid Slime, Sneeze, Gas Cloud, Chomp.	Siegebreaker - Triple Hit, Charge, Leg Stomp, Grab and Throw.	Cydaea - Pain Bolts, Web Trap, Spider Pits.	Azmodan - Bellyflop, Laser Attack, Falling Corpses, Demon Gate, Pool of Destruction, Globe of Annihilation.
Act Four Uniques	Rakanoth - Cleave, Teleport Strike, Blade Strike, Volley, Summon Minion.	Izual - Frost Nova, Frozen, Summon Minions.	Diablo - Phase One: Claw Rip, Charge, Hell Spikes, Shadow Vanish Grab, Curse of Destruction, Ring of Fire, Ground Stomp, Curse of Anguish, Curse of Hatred. Phase Two: Shadow Clone. Phase Three: Overdrive, Lightning Breath.