Armored by Faith. While the archpriest is wearing no armor and wielding no shield, its AC includes its Wisdom modifier.

Divine Weapons. The archpriest's melee weapon attacks are magical, and it uses its Wisdom modifier, instead of Strength or Dexterity, for the attack and damage rolls. When the archpriest hits with a melee attack using a weapon, the weapon deals an extra 2d8 radiant damage (included in the attack). 
    As a bonus action, the archpriest can expend a spell slot to increase this radiant damage by 2d8 until the end of the turn. If the archpriest expends a spell slot of 2nd level or higher, the radiant damage increases by an additional 1d8 for each level above 1st.

Spellcasting. The archpriest is a 19th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 17, +9 to hit with spell attacks). It has the following cleric spells prepared.

Cantrips (at-will): guidance, light, renewal (new), sacred flame, spare the dying
1st level (4 slots): bless, cure wounds, detect evil and good, sanctuary
2nd level (3 slots): hold person, lesser restoration, spiritual weapon
3nd level (3 slots): daylight, dispel magic, revivify
4th level (3 slots): banishment, divination, guardian of faith
5th level (3 slots): commune, flame strike, greater restoration, hallow, mass cure wounds
6th level (2 slots): heal, true seeing
7th level (1 slot): divine word
8th level (1 slot): holy aura
9th level (1 slot): mass heal, true resurrection

Turning. Undead have disadvantage on attack rolls made against the archpriest. If an undead can see or hear the archpriest, it must spend 2 feet of movement for every 1 foot it moves when moving closer to the archpriest. Any undead that attacks the archpriest while within 5 feet of it takes 9 (2d8) radiant damage on a hit or miss. An undead with a challenge rating of 2 or less that ends its turn within 30 feet of the archpriest dies instantly. While the archpriest is incapacitated, this trait does not function.

Quarterstaff. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 8 (1d6 + 5) bludgeoning damage, or 9 (1d8 + 5) bludgeoning damage if used with two hands, plus 9 (2d8) radiant damage.

Holy Touch (3/Hour). The archpriest touches a creature. If the target is not undead, it magically regains 22 (4d8 + 5) hit points, and the archpriest removes any disease and neutralizes any poison afflicting the target. If the target is an undead, it must instead succeed on a DC 15 Dexterity saving throw or else take 22 (4d8 + 5) radiant damage. Constructs are immune to this ability.