Amphibious. Zereni can breathe air and water.

Legendary Resistance (3/Day). If Zereni fails a saving throw, she can choose to succeed instead.

Change Shape (1/Day). Once a day zereni can magically shifts into her humanoid form, that of Madame Zereni, a Human Wizard. 

Her statistics and capabilities are replaced by those of the new form.

Spellcasting. Zereni casts one of the following spells, requiring no material components and using Intelligence as the spellcasting ability:

1/day each: control winds, divination, ice storm, wall of force

Multiattack. Zereni makes three attacks: one with her bite and two with her claws.

Bite. Melee Weapon Attack:  +16 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) piercing damage plus 6 (1d12) thunder damage.

Claw. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 16 (2d6 + 9) slashing damage.

Draconic Blessing. Zereni utters a few inspiring words to one creature she can see within 30 feet of her. If the creature can hear her, it can add a d10 to one attack roll or saving throw it makes before the start of Zereni’s next turn.

Breath Weapons (Recharge 5–6). Zereni uses one of the following breath weapons.

Lightning Breath. Zereni exhales lightning in a 120-foot line that is 10 feet wide. Each creature in that line must make a DC 23 Dexterity saving throw, taking 55 (10d10) lightning damage on a failed save, or half as much damage on a successful one.

Repulsion Breath. Zereni exhales repulsion energy in a 30-foot cone. Each creature in that area must succeed on a DC 23 Strength saving throw. On a failed save, the creature is pushed 60 feet away from the dragon.

Negate Spell (2/Day). When Zereni sees a creature within 60 feet of her casting a spell, she tries to interrupt it. If the creature is casting a spell using a spell slot of 8th level or lower, its spell fails and has no effect. If it is casting a 9th-level spell, it must succeed on a DC 22 Intelligence, Wisdom, or Charisma saving throw, or the spells fails and has no effect.

The dragon 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. The dragon regains spent legendary actions at the start of its turn.

Bite. Zereni makes a bite attack.

Draconic Blessing (Costs 2 Actions). Zereni uses draconic blessing.

Elder Tempest (Costs 3 Actions). Zereni, the Elderfang magically unleashes the wrath of the tempest. Creatures within 60 feet of Zereni, including ones behind barriers and around corners must make a DC 23 Constitution saving throw or take 44 (8d10) lightning damage, or half as much damage on a successful one.