So, reviewing the rules of Belial, he has all the standard Deathwing rules (Fearless, Hatred (CSM), Grim Resolve), plus some bonuses to fighting in challenges and the option to use a Relic Sword (Sword of Silence — S: User AP3 Fleshbane, Master-crafted)
What really stands out about him, though, is the Tactical Precision rule which states “Belial and his unit do not scatter when arriving by Deep Strike.”
As you can see from the rule, there is no restriction on what unit Belial is joined to, and thus no restriction on what friendly units can benefit from his “no scatter” deep strike ability.
This opens up numerous tactical possibilities across all armies that can ally with Dark Angels as Battle Brothers, specifically all the Imperium codices/supplements. Below is a list of allied units/formations that could benefit from having Belial attached and conferring his “Tactical Precision” rule when they come in via Deep Strike:
(1) – Ultramarines Strike Force Ultra — Attach Belial and Marneus Calgar to one of the 10 man Tactical Terminators squads equipped with dual Heavy Flamers, then Deep Strike in with no scatter and release 4 Flamer templates and 24 Storm Bolter shots (Strike Force Ultra allows the formation add one more shot to all weapons when the unit arrives by DS), all with Rending (thanks to Calgar using the Storm of Fire WL Trait)… even Wraithknights will have trouble absorbing that kind of firepower and “normal” infantry will just evaporate… plus you will have a massive Terminator squad right where you want it in the enemy’s face turn 1, thanks to no scatter DS.
(2) – Deathwing Knights and attached Brother-Captain Stern — While they cannot put out any shooting the turn they come in, they are super tough with the potential for 2++ saves and T5 all around, thanks to Sanctuary from Stern and the Fortress of Shields rule. They should be able to absorb all kinds of enemy shooting (even Grav weapons, with a 2++ save), and then charge turn 2, using Hammerhand from Stern to strike with WS5 in either “Normal mode” S8 AP3 I4 or “Smite mode” S10 AP2 I4 (but only one attack per model), which will bring down just about anything they encounter.
(3) – Wolf Guard Terminators with Combi-meltas and attached TDA Rune Priest — The ability to land EXACTLY where you want is invaluable for this combo, as it means you can put the WG Terminators precisely in melta range of whatever high-value target you want to bring down (i.e. Imperial Knight, Monstrous/Gargantuan Creatures, AV14 vehicles, Grav Cannon Centurions, etc.). You then use the Rune Priest to cast Prescience on the Terminators, twin-linking their meltas (and making them better in assault if they are charged in the next turn) for even more reliable destruction.
(4) – GK Paladins with 4 x Special Weapons — Being able to put a squad of Paladins exactly where you want them provides exceptional flexibility and offensive capacity, whether you equip them with four Incinerators and “burninate” down a unit with multiple S6 AP4 templates or four Psycannons and cut loose with 16 x S7 AP4 Rending shots into a vehicle squadron’s rear armor. On top of this, like with Strike Force Ultra, DW Knights, and WG Terminators, you also have a dangerous CC unit right on the enemy’s doorstep with no risk of scatter.
5 – GK Terminators/Strike Squads/Interceptors with dual Incinerators — A cheaper, more “economy” version of the Paladin build above, this still is a viable battle group that can put multiple Incinerator templates and lots of SB fire right where you need it, plus provide lots of threat from force weapons in the next turn, with Belial attaching to the squad and bringing them in precisely.
6 – Sanguinary Guard with multiple Inferno Pistols and JP Sanguinary Priest — A combo that is perhaps not very conventional, this unit benefits greatly from being able to get all of its Inferno Pistols within “3” melta range because of Belial’s DS accuracy and then, in subsequent turns, use its superior 2+ armor and 12″ Jump Pack moves to continue to deal significant damage to the enemy lines.
7 – JP Death Company with attached Astorath — Similar to the Sanguinary Guard, you can add Inferno/Plasma pistols or Hand Flamers to taste, but the real benefit is having up to 15 bloodthirsty Death Company models on your opponent’s doorstep, ready to charge with 5A per model and re-rolls to Hit/to Wound, thanks to Astorath’s buffs.
8 – JP Vanguard Vets gun slinging massed Grav Pistols and attached JP Librarian — A more mobile alternative to Grav Centurions or Grav Bikers, you can potentially have up to 20 Grav shots at 12″ with this battle group, all twin-linked via Prescience from the attached Librarian.
9 – Blood Angel JP Assault Squad with dual Special Weapons and dual pistols on Sgt — With only 5 members, this squad can pack quite a bunch of specialized firepower, be it two flamers and two hand flamers <or> two meltas and two inferno pistols <or> two plasma guns and two plasma pistols/Grav pistols. Any way you do it, Belial can bring this squad in precisely and deliver all the firepower exactly where it needs to be.
10 – Militarum Tempestus Command Squad with 4 x Special Weapons — You have multiple options here, including Flamers, Grenade Launcher, Meltaguns, HS Volley-guns, and Plasma Guns (plus a Plasma Pistol for the Tempestor Prime, if you want it), all of which combine well with Orders that either Twin-Link the shooting or give the squad Preferred Enemy. Add in the fact that the squad has Move through Cover and you have Belial Deep Strike them into terrain with no scatter and no risk of Dangerous Terrain tests, significantly improving their survivability as they deliver all that shooting exactly where you need it.
11 – Sisters of Battle Seraphim with 4 x Inferno Pistols/Hand Flamers and attached Celestine — Another great utility unit that can be configured for clearing hordes (Hand Flamers/BPs and Celestine’s Heavy Flamer) or bringing down vehicles/MCs (Inferno Pistols). On top of this, the whole unit has Hit and Run and, between Belial and Celestine, is actually quite a dangerous close combat threat in subsequent turns.
12 – Cult Mechanicus “Holy Requisitioner” formation — A bit of an unusual battle group here, this formation consists of a Tech-Priest Dominus and 2-3 Kataphron Breacher squads and gives them all the ability to Deep Strike. Additionally, they all come in on a single Reserve Roll (which can be re-rolled if desired) and the Breachers do not scatter if they DS within 6″ of the Tech-Priest Dominus. Now, the Tech-Priest can “organically” deep strike with no scatter, provided he lands within 6″ of an Objective, but attaching Belial to him means he has the option to deep strike ANYWHERE, objective or not, and then bring in the Breachers around him with no scatter, making for a much more flexible battle group. Once they arrive, the Breacher can put out significant anti-vehicle firepower with Heavy Arc Rifles (i.e. 2 shot Haywire weapons) or anti-MC/TEQ shooting with Torsion Cannons (S8 AP1 shots that inflict D3 wounds for each unsaved wound or D3 HPs lost for each penetrating hit). Also, while not exceptional, the Breachers have decent close combat capabilities for future turns, especially if they take the Hydraulic Claw upgrade for S10 AP2 attacks.