Baelgorath, Ledger-Keeper of Souls

Diabolical Restoration. If the pit fiend dies outside the Nine Hells, its body disappears in sulfurous smoke, and it gains a new body instantly, reviving with all its Hit Points somewhere in the Nine Hells.

Fear Aura. The pit fiend emanates an aura in a 20-foot Emanation while it doesn’t have the Incapacitated condition. Wisdom Saving Throw: DC 21, any enemy that starts its turn in the aura. Failure: The target has the Frightened condition until the start of its next turn. Success: The target is immune to this pit fiend’s aura for 24 hours.

Legendary Resistance (4/Day). If the pit fiend fails a saving throw, it can choose to succeed instead.

Magic Resistance. The pit fiend has Advantage on saving throws against spells and other magical effects.

Soul-Bound Ledger. Baelgorath always knows the location (plane and rough distance) of any creature bound by infernal contract to his ledger. He can cast Locate Creature at will against them, ignoring planar boundaries.

Contractual Chains. Any creature that owes Baelgorath a contract has disadvantage on saving throws against his magic.

Multiattack. The pit fiend makes one Bite attack, two Devilish Claw attacks, and one Fiery Mace attack.

Bite. Melee Attack Roll: +14, reach 10 ft. Hit: 18 (3d6 + 8) Piercing damage. If the target is a creature, it must make the following saving throw. Constitution Saving Throw: DC 21. Failure: The target has the Poisoned condition. While Poisoned, the target can’t regain Hit Points and takes 21 (6d6) Poison damage at the start of each of its turns, and it repeats the save at the end of each of its turns, ending the effect on itself on a success. After 1 minute, it succeeds automatically.

Devilish Claw. Melee Attack Roll: +14, reach 10 ft. Hit: 26 (4d8 + 8) Necrotic damage.

Fiery Mace. Melee Attack Roll: +14, reach 10 ft. Hit: 22 (4d6 + 8) Force damage plus 21 (6d6) Fire damage.

Hellfire Spellcasting (Recharge 4–6). The pit fiend casts Fireball (level 5 version) twice, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 21). It can replace one Fireball with Hold Monster (level 7 version) or Wall of Fire.

Soul Rend (Recharge 5-6). Baelgorath targets a creature within 60 ft bound by infernal contract. DC 21 Cha save or take 45 (10d8) necrotic and be paralyzed until the end of Baelgorath's next turn.

Infernal Command. Baelgorath orders one devil within 60 ft. to make an attack or move up to its speed immediately.

Ledger Clause. When a creature within 60 ft. casts a spell, Baelgorath can force them to make a DC 21 Cha save. On a fail, the spell fizzles but is still expended. 

Baelgorath can take 3 legendary actions:

• Fiery Strike. Make one Fiery Mace attack.

• Infernal Glare. Force one creature within 30 ft. to make a DC 21 Wis save or be Frightened until the end of its next turn.

• Hellfire Burst (Costs 2). Cast Fireball (5th-level) centered on a point within 120 ft.

• Summon Devil (Costs 3). Summons 1d4 bearded devils or 1 horned devil.