Tiamat's Blessing. The high priestess of Tiamat's AC includes her Wisdom modifier.

Blood of Tiamat. The high priestess of Tiamat has immunity to acid, cold, fire, lightning, and poison damage.

Divine Power. The high priestess of Tiamat has a 5 percent chance every turn to regain a spell slot of 8th level or lower. Roll a d20 at the start of each of her turns. On a one, the high priestess regains a spell slot. Roll a d8 to determine what level spell slot.

Spellcasting. The high priestess of Tiamat is a 20th-level spellcaster. Her spellcasting ability is Wisdom (spell save DC 22, +9 to hit with spell attacks). The high priestess has following cleric spells prepared:
Cantrips (at will): sacred flame, word of radiance, toll the dead, thaumaturgy, resistance
1st level (4 slots): guiding bolt, command

2nd level (3 slots): spiritual weapon, blindness/deafness 

3rd level (3 slots): bestow curse, glyph of warding 

4th level (3 slots): banishment, death ward 

5th level (3 slots): contagion, flame strike, planar binding

6th level (2 slots): blade barrier, harm, heal

7th level (2 slots): fire storm, symbol

8th level (1 slots): holy aura, antimagic field

9th level (1 slots): gate, true resurrection 

Multiattack. The high priestess of Tiamat can use her Chromatic Breath. She then makes two attacks, any of which can be replaced by a spell.

Dragonsaber. Melee Weapon Attack: +11 to hit, reach 5 ft., 1 target. Hit: 10 (1d10 + 5) slashing damage + 10 (3d6) acid, cold, fire, lightning, or poison damage. 

Chromatic Breath (Recharge 6). The high priestess of Tiamat exhales energy, choosing acid, cold, fire, lightning, or poison for the type of damage.

1. Acid Breath (90 foot line). Each creature in the line must make a DC 22 Dexterity saving throw, taking 10d8 acid damage on a failed save, or half as much on a successful one.

2. Cold Breath (90 foot cone). Each creature in the cone must make a DC 22 Constitution saving throw, taking 10d8 cold damage on a failed save, or half as much on a successful one.

3. Fire Breath (90 foot cone). Each creature in the cone must make a DC 22 Dexterity saving throw, taking 10d8 fire damage on a failed save, or half as much on a successful one.

4. Lightning Breath (90 foot line). Each creature in the line must make a DC 22 Dexterity saving throw, taking 10d8 lightning damage on a failed save, or half as much on a successful one.

5. Poison Breath (90 foot cone). Each creature in the line must make a DC 22 Constitution saving throw, taking 10d8 poison damage on a failed save, or half as much on a successful one.