Belial is the Grand Master of the Deathwing, or First Captain of the Dark Angels Space Marine Chapter. While getting less screen time (page time?) than Azrael and Sammael, Belial is perhaps the most popular HQ choice a Dark Angels Army can field. This is possibly because Dark Angels players are as as flagellant as 40k players come, and Belial is the most emo motherfucker in an army known for being emo. That, or terminators are fucking cool and fielding armies of nothing but them is awesome. It makes us all mad fucking Blood Angel spacevampirefags get all the glory in Space Hulk. (Until, of course, Space Hulk : Deathwing, which is already known as being one of the absolute fucking best 40k games to date.)
His safe word is "Et spiritu vexatus", No loyalist chapter would ever resort to hypno code words to keep their troops in line.
"Azrael, i'm gonna get motion sickness again."
- – Belial in TTS
Belial (which is a Hebrew idiom for a "worthless man", a name which could not be more ironic) has a career that would fill volumes if the Dark Angels kept such records, due to
most of it being about hunting the Fallen Angels A LIFETIME OF LOYAL SERVICE TO THE EMPEROR--EXTRA EMPHASIS ON THE LOYAL PART--BEING ITS OWN REWARD.
Raised on the Feudal World of Bregundia, born the son of the town magister, he was forbidden from joining the tournaments of hopeful children, competing to earn a place in the Dark Angels when the recruiters came. So he did what any sensible person would do: completely ignore his father and steal the armour of a Knight to compete in the games, where he kicked everyone's asses. Then, when the Dark Angels' chaplain ordered him to remove his helmet and reveal himself, Belial refused (for fear of upsetting his father... WAY to go kid!) and declared he would rather fight the chaplain instead. Surprisingly, the duel lasted for SEVEN minutes before Belial inevitably lost. An AMAZING feat regardless, as Belial was ELEVEN YEARS OLD whilst the Chaplain was, well, a Space Marine Chaplain. Thus was a new recruit secured for the Dark Angels anyway. Years later when Belial had completed his induction to the chapter and was promoted to full battle brother, his first act was to challenge the chaplain to a rematch, which he won in under a minute. (That being said, the Chaplain probably went easy on him since even the Dark Angels, except Asmodai, don't murder their aspirants for no reason.)
Belial steadily raised his way through the ranks and secured himself a place in the Deathwing where he was Brother-Captain Azrael's second in command when he was in charge of the Deathwing. After the death of Supreme Grand Master Naberius, as 1st Captain, Azrael assumed command of the entire chapter needed to appoint someone to the vacant captaincy position, so he asked Belial if he had any ambitions to command, where Belial replied that he was content enough to serve, therefore Azrael elevated some dude called Lanval to the Master of the Deathwing and granted him the Sword of Silence (the sword meant to honour the Chapter's greatest warrior) which meant that Belial remained a Brother-Sergeant.
Presumably Lanval didn't keep Belial on as second in command like Azrael did and within a few years Belial would move out of the Deathwing and take control of a squad in the Third Company. There his squad were renowned as ruthless perfectionists; where every missed bolter shot or parried blow is seen as a failure equal to losing the entire battle. Among his most famous exploits were the destruction of the Black Crusade of Furion, personally killing the Chaos Lord of Khorne in single combat, earning his promotion to Company Master of the Third Company and the Sword of Silence (guess Lanval died or gave it up), being given to him by Azrael.
Eventually, the Third Company was on layover at Piscina IV after the Rock had picked up load of aspirants from neighboring Piscina V when WAAAGH Ghazghkull showed up and proceed to lay waste to everything it could find. For two weeks Belial and the Third Company managed to hold off the Beast of Armageddon until the Rock returned with the rest of the Chapter. It was by unanimous decision that Belial would come back to the Deathwing and be promoted to its Grand Master due to the death of the previous Grand Master, his reputation as a harsh and perfecting taskmaster convinced the Inner Circle that no-one else was better suited for the job.
Belial's rules pay lip-service to the fact that he is the Dark Angels' greatest warrior; he has an extra attack over a normal Captain and rerolls to-hit in challenges. But that doesn't mean a great deal considering the fact that they are mostly a shooty army that are better at supporting one another rather than playing Herohammer... he's no match for Lysander or Calgar and really needs a Deathwing squad to keep him alive, and anyone built for challenges will still likely just Instant Death him. His warlord trait allows him precision shots and for his storm bolter to ignore cover. He has a rule preventing him from scattering when arriving from deep strike. In addition he comes with a teleporter homer built in, making him perfect for setting up further deep strikes. His Sword of Silence, his base weapon, has Fleshbane letting him wound with the ease of a Thunderhammer at regular initiative. In addition he can switch out his weapons for lightning claws, or a thunderhammer and stormshield for free. With the claws you're trading 2+ wounding for re-rolls and +1A. The hammer and shield is usually a safe bet; while the sword strikes faster the hammer lets you instant death a lot of stuff with the safety of a 3++. Belial's big downside is that for 190 points, we were really hoping for something more impressive than a Fleshbane, 3-wound Termie Sergeant. Just a small reminder that a Terminator captain doesn't cost a lot less than this though.
Belial is still geared towards being an excellent duelist this edition. The Sword of Silence still wounds non-vehicles on a 2+, but also has S:+1 which means hes gets to wound those T8-T9 vehicles on a 5+. While he can still swap his weapons for a a TH/SS or pair of claws, the Sword has the stats AP:-3 Damage:2 so its role has a TEQ slayer has come to the fore, where it simply failed in the past. It also reduces the WS of any attacks coming his way by 1, a handy little buff considering that all those guys with Fists and Hammers already reduced their WS by another 1. Belial should now really only choose the claws if he has no intention of fighting characters and needs to focus on large groups, while the Thunder Hammer should be taken if you really need the 3+ invulnerable save from the Storm Shield, since it even the Sword beats it for wounding potential against Toughness 5 or higher, thanks to a the new wounding table. Belial also buffs his army a little bit: Dark Angels units within 6 inches reroll to-hit rolls of 1, or all failed to-hit rolls if they're part of Deathwing. Do note that his other big draw, not scattering on Deep Striking, is universal through the game now, so he's lost a bit of uniqueness if not the power that came with it.