Homebrew
Innate Spellcasting. Bel's spellcasting ability is Charisma (spell save DC 22). He can innately cast the following spells, requiring no material components:
Hear Name. Bel hears his name when spoken, regardless of distance or planar boundaries. If a creature speaks the name “Bel” aloud thrice within 1 minute (hereafter "the caller"), Bel learns the caller's name, precise location, and intent (i.e., they spoke in reverence, fear, mockery, etc.). Bel can also perceive through the speaker's senses for up to 60 rounds (10 minutes) while remaining aware of his surroundings. The connection ends early if voluntarily closed by Bel, requiring no action. Once per long rest, Bel can brand a caller who fails a DC 22 Charisma save with a rune visible only to celestials, elementals, fey, fiends, and undead for 1d4 hours. Bel chooses whether the brand is a boon or bane. If a boon, the caller has advantage on Dexterity checks and extends the duration of Invisibility or Evasive spells like blink while marked; if a bane, the caller has disadvantage on Dexterity checks and is unable to benefit from invisibility or teleportation effects for the duration.
Special Equipment, Hellfire Mace. Bel wields a mace with the Hellfire weapon property.
Fear Aura. Any creature hostile to Bel that starts its turn within 20 feet of him must make a DC 22 Wisdom saving throw unless Bel is incapacitated. If the save fails, the creature has the frightened condition until the start of his next turn. If a creature's saving throw is successful, the creature is immune to Bel's Fear Aura for the next 24 hours.
Magic Resistance. Bel has advantage on saving throws against spells and other magical effects.
Magic Weapons. Bel's weapon attacks are magical.
Vigilant. Bel cannot be surprised and has advantage on initiative rolls.
Multiattack. Bel attacks twice with either his Claw or his Hellfire Mace in any combination and once with his Tail.
Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 8) slashing damage.
Hellfire Mace. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: (2d6 + 7) bludgeoning damage plus 21 (6d6) fire damage. This weapon is fashioned from infernal iron and traced with veins of hellfire that shed dim light in a 5-foot-radius. Any humanoid killed by an attack made with this weapon has its soul funneled into the River Styx, where it's reborn instantly as a lemure devil.
Tail. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 24 (3d10 + 8) bludgeoning damage.
Unseen Feint. Bel teleports up to 60 feet to an unoccupied space he can see and becomes invisible until the start of his next turn or until he makes an attack. The next attack he makes while invisible deals an extra 21 (6d6) psychic damage.
Kill Order (Recharge 5-6). Bel targets up to three creatures he can see within 60 feet. Each must make a DC 22 Wisdom saving throw. On a failed save, a target is charmed for 1 minute. On its turn, a creature charmed in this way must use its action to attack the nearest creature, favoring Bel's enemies if possible. A target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. Creatures immune to the charmed condition automatically succeed on the saving throw.
Cunning Strategist. Bel may Dash, Disengage, Help, or Hide as a bonus action.
Deadly Orders. Bel commands his troops to prioritize attacking a specific foe. Creatures allied with Bel are more likely to target the designated foe, and their Attack rolls against it have Advantage. The Deadly Orders remain in effect until the start of Bel's next turn.
Supreme Strategist. Bel may swap the position of any two creatures he can see within 120 feet of each other. To be an eligible target, a creature must be willing, charmed, frightened, or incapacitated.
Tactical Mastermind. Bel can take 3 reactions per round, but only one per turn.
Counterplay. When a creature within 60 feet of Bel that he can see makes an attack, Bel can impose disadvantage on the attack roll.
Predictive Evasion. When an ally within 30 feet is targeted by an area effect, Bel can move that ally up to 10 feet as a reaction without provoking attacks of opportunity.
Spell Deflection. When a creature within 60 feet of Bel that he can see casts a spell, Bel and the caster roll opposed spellcasting ability checks. Bel uses Charisma for this roll. If Bel wins the contest, the spellcaster must choose a different target(s) or area of effect that does not overlap the original target(s) or area of effect.
Legendary Action Uses: 3. Immediately after another creature's turn, Bel can expend a use to take one of the following actions. Bel regains all expended uses at the start of each of their turns.
Fireball. Bel casts Fireball.
Strategic Repositioning. Bel moves up to his speed without provoking opportunity attacks.
Demoralize (Costs 2 Actions). Bel unleashes an insult laced with subtle enchantments and makes a Hellfire Mace attack against a target marked by Deadly Orders. If Bel's attack hits the target, allies of the target must make a DC 22 Wisdom saving throw or have a 1d4 penalty on one attack roll before the end of their next turn. If Bel's attack misses the target, the target's allies instead have a 1d4 bonus on one attack roll before the end of their next turn.
While within the Zephon Citadel, Bel can invoke its ambient power to take lair actions. On initiative count 20 (losing initiative ties), Bel can take a lair action to cause one of the following effects; he can't use the same effect two rounds in a row:
Battlefield Command. Bel chooses up to 3 willing creatures that can hear hear him within 120 feet. Each can move up to half its speed and make one weapon attack.
Blaze of Battle. Gouts of hellfire erupt from the ground. Each enemy within 20 feet of a point Bel chooses must succeed on a DC 22 Dexterity saving throw or take 14 (4d6) fire damage and be frightened until the end of its next turn.
Propaganda. Up to 4 creatures of Bel's choice within 120 feet of him who can hear him must make a DC 22 Intelligence saving throw. On a failed save, a target takes 18 (4d8) psychic damage and is charmed until the end of its next turn.