Caelius Benedictus, the Instrument of Decay

Death's Inescapability. Every time Caelius uses his Draining Necrosis feature to absorb a soul, he gains a cumulative -1 penalty to his saving throws. If Caelius absorbs 3 souls, his movement speed is lowered by 10 feet. If Caelius absorbs 6 souls, his armor class is lowered by 3. If Caelius absorbs 10 souls, he can no longer take Bonus Actions, Reactions, and Legendary Actions.

Frail Body. Caelius has disadvantage on Strength ability checks and saving throws. 

Necrotic Absorption. Whenever Caelius is subjected to necrotic damage, he takes no damage and instead regains a number of hit points equal to the necrotic damage dealt.

Spellcasting. Caelius is a 8th-level spellcaster. His spellcasting ability is Charisma (spell save DC 16, +9 to hit with spell attacks). Caelius has the following spells prepared:
Cantrips (at will): Firebolt, Hellfire
1st level (# slots): Command, Hellish Rebuke, Inflict Wounds
2nd level (3 slots): Infernal Challenge, Scorching Ray
3rd level (2 slots): Bestow Curse, Fireball
4th level (2 slots): Blight

Multiattack. Caelius makes two attacks when he takes the Attack action. Alternatively, he may instead replace one attack with the casting of one spell, a use of his Draining Necrosis feature, or, if available, a use of his Necrotic Conduit feature.

Longsword of Justice. Melee Weapon Attack: +9 to hit, reach 5 ft., 1 target. Hit: 20 (2d12 + 7) slashing damage plus 5 (2d4) necrotic damage.

Draining Necrosis. Caelius uses his Spirit Jar to absorb the soul of an ally within 5 feet of him. He regains 2d10 hit points, and has advantage on his next attack roll, ability check, or saving throw until the end of his next turn. 

Necrotic Conduit (Recharge 5-6). Caelius touches a hostile creature within 5 feet of him. That creature must make a Constitution saving throw. The target takes 11 (2d10) necrotic damage, or half as much on a successful save, and Caelius regains an amount of health equal to the damage dealt.

Tactical Maneuver. Caelius moves up to 10 feet without provoking opportunity attacks.

Parry. Caelius Benedictus adds 3 to his AC against one melee attack that would hit him. To do so,  he must see the attacker and be wielding a melee weapon. If the attack misses, he can make a melee attack against the attacker using his Longsword of Justice.

Caelius 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. Caelius regains spent legendary actions at the start of his turn.

Attack (Costs 1 Action). Caelius makes an attack with this Necrotic Blade or casts a Cantrip.
Recharge (Costs 1 Action). Caelius makes a roll to recharge his Necrotic Conduit feature.
Tactical Maneuver (Costs 1 Action). Caelius moves up to 10 feet without provoking opportunity attacks.
Draining Necrosis (Costs 2 Actions). Caelius uses his Draining Necrosis feature.
Spellcasting (Costs 2 Actions). Caelius casts a spell using a spell slot.