Homebrew
Innate Spellcasting: Belial's spellcasting ability is Charisma (spell save DC 20). He can innately cast the following spells, requiring no material components:
Magic Resistance: Belial has advantage on saving throws against spells and other magical effects. Legendary Resistance (3/Day) If Belial fails a saving throw, he can choose to succeed instead. Abyssal Presence: Creatures within 30 feet of Belial must succeed on a DC 20 Wisdom saving throw or become frightened for 1 minute. A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Belial's Abyssal Presence for the next 24 hours.
Multiattack. Belial makes three attacks: one with his bite and two with his claws. Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 22 (3d8 + 6) piercing damage and 14 (4d6) poison damage. Claw. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 18 (2d8 + 6) slashing damage. Tail. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 6) bludgeoning damage. Summon Imps (1/Day). Belial summons 2d4 imps to fight for him. They appear within 30 feet of him and act on their own initiative.
Belial can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time, and only at the end of another creature's turn. Belial regains spent legendary actions at the start of his turn. Attack. Belial makes one claw or tail attack. Cast a Spell (Costs 2 Actions). Belial casts a spell from his Innate Spellcasting ability. Teleport. Belial magically teleports, along with any equipment he is wearing or carrying, up to 120 feet to an unoccupied space he can see.