Cult Plague Shepherd

Master of the Infected. The Shepherd can control up to four Plaguewretches within 120 feet.

Controlled Plaguewretches understand its commands regardless of language.

Shared Infection. The Shepherd always knows the direction and approximate distance of its controlled Plaguewretches while they are within 1 mile.

This gives you an important story mechanic: capturing one alive potentially allows the party to discover that the creatures aren't acting independently.

Plague Immunity. The Shepherd automatically succeeds on saving throws against the plague strain it uses to create its servants.

Multiattack. The Shepherd makes two Plague Dagger attacks.

Plague Dagger. Melee or Ranged Weapon Attack: +5 to hit, reach 5 ft. or range 20/60 ft., one creature. Hit: 5 (1d4 + 3) piercing plus 3 (1d6) poison damage.

Command the Afflicted. The Shepherd chooses up to two controlled Plaguewretches within 60 feet. Each can immediately use its reaction to move up to half its speed and make one Diseased Claw attack.

This movement doesn't provoke opportunity attacks.

Burst the Infection (Recharge 5–6). The Shepherd chooses one controlled Plaguewretch within 60 feet.

The creature's crystalline growths violently rupture.

Every creature within 10 feet of the Plaguewretch makes a DC 13 Constitution save, taking 14 (4d6) poison damage on a failure or half on a success.

The Plaguewretch takes 7 (2d6) necrotic damage.

This immediately tells the players something disturbing:

The cultist considers the infected disposable.

Focus the Pack. The Shepherd chooses one creature it can see within 60 feet. Until the start of the Shepherd's next turn, controlled Plaguewretches have advantage on their first attack against that creature.

Take the Pain. When a controlled Plaguewretch within 30 feet takes damage, the Shepherd can reduce that damage by 1d8 + 2. The Shepherd takes necrotic damage equal to the amount prevented.