Halo of Pestilence.

The champion is surrounded by an aura of deadly magic that takes the form of a host of glowing white insects. Each creature that starts its turn within 10 feet of the champion must succeed on a DC 18 Constitution saving throw or have the incapacitated condition until the start of its next turn, as the insects ravage its body.

Legendary Resistance (3/Day).

If the champion fails a saving throw, it can choose to succeed instead.

Actions

Multiattack.

The champion makes two Blight Staff attacks, two Plague Bolt attacks, or one of each.

Blight Staff.

Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) bludgeoning damage plus 21 (6d6) necrotic damage.

Plague Bolt.

Ranged Spell Attack: +10 to hit, range 120 ft., one target. Hit: 23 (4d8 + 5) poison damage, and the target has the poisoned condition until the start of the champion's next turn.

Spellcasting.

The champion casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 18):

At will: Detect Magic, Mage Armor (self only)
2/day each: Blight, Blindness/Deafness
1/day each: Cloudkill, Contagion

Legendary Actions

The grim champion of pestilence can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The grim champion of pestilence regains spent legendary actions at the start of its turn.

Attack. The champion makes one Blight Staff or Plague Bolt attack.
Furious Pursuit. The champion moves up to its speed or commands its mount to move up to its speed. This movement doesn't provoke opportunity attacks.
Cast a Spell (Costs 2 Actions). The champion uses Spellcasting.

Grim Champion of Pestilence