Antonius Vasseret

Discorporation. When Antonius drops to 0 hit points or dies, his body is destroyed but his essence travels back to Tiamat's domain in the Nine Hells, and he is unable to take physical form for a time. 

Legendary Resistance (5/day). If Antonius fails a saving throw, he can choose to succeed instead. 

Magic weapons. Antonius' weapon attacks are magical. 

Reaper. When Antonius casts a necromancy spell that normally targets only one creature, the spell can instead target two creatures within range and within 5 feet of each other. 

Spellcasting. Antonius is a 12th-level spellcaster. His spellcasting ability is Wisdom (spell save DC 14, +6 to hit with spell attacks). Antonius has the following Cleric spells prepared:
1st level (4 slots): bane, shield of faith, guiding bolt
2nd level (3 slots): ray of enfeeblement, blindness/deafness
3rd level (3 slots): animate dead, vampiric touch,  false life
4th level (3 slots): banishment, blight, death ward, guardian of faith
5th level (2 slots): flame strike, contagion
6th level (1 slot): cloudkill

Multiattack. Antonius uses Ray of Sickness twice.

Ray of Sickness. Ranged Spell Attack: +6 to hit, reach 60 ft., one target. Hit: 10 ((1d8) poison damage. 

Toll the Dead. Ranged Spell Attack, reach 60 ft., one target. The target must succeed on a Wisdom saving throw or take (2d8) necrotic damage. If the target is missing any of its hit points, it instead takes (2d12) necrotic damage. 

Solemnity, Blade of Tiamat. Melee Weapon Attack:+5 to hit, reach 5 ft., one target. Hit: (1d8 + 2) slashing damage. The target must make a Constitution saving throw. The creature takes (2d10) necrotic damage on a failed save, or half has much damage on a successful one. 

Enter the description for how bonus actions work for your monster here.

Bolstered Resolve. When taking damage from any damage immunities, the damage that would have been dealt is instead healed. If at max hit points, it will be added as temporary hit points. 

Antonius 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. Antonius regains spent legendary actions at the start of his turn, unless the spell is still active:

Spirit Guardians. He calls forth spirits to protect him. They flit around him at a distance of 15 feet for two turns. When he casts this spell, he can designate any number of creatures he can see to be unaffected by it. An affected creature's speed is halved in the area, and when the creature enters the area for the first time on a turn or starts its turn there, it must make a Wisdom saving throw. On a failed save, the creature takes (3d8) necrotic damage. On a successful save, the creature takes half as much damage. 

Inflict Wounds. He makes a melee spell attack against a creature he can reach. On a hit, the target takes (2d6) necrotic damage.

When Antonius reaches 50% and 25% of his original health, he can take one mythic action, choosing from the options below. A mythic action can only be used once and at the end of another creature's turn. Antonius regains spent legendary actions after 1 week has passed. 

Summon Celestial. He calls forth a celestial spirit. It manifests in a hellish form in an unoccupied space that Antonius can see within range. This corporeal form uses Celestial Spirit stat block and is an Avenger. This creature is an ally to him and all his companions. In combat, the creature shares his initiative count, but it takes its turn immediately after his. 

Divine Intervention. He calls upon the aid of Tiamat herself. Any creature with the curse Will of Tiamat succumb to the whispers, losing control of their actions until they break the curse or Tiamat no longer needs the aid of the creature.