Virizan, Exarch of Tiamat (Green)

Amphibious. Virizan can breathe air and water.

Exarch’s Shape. Virizan changes to one of his other form – venom dragon, naga, snake swarm or snaketongue. The form must have more than 0 hit points.  Unless otherwise noted he has the same abilities, saving throws, skills, senses, immunities, traits, reactions and legendary actions in all forms.

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

Shifting Body. Track hit points separately for each of Virizan’s forms. He is bloodied (at 50% health) once two forms have dropped to 0 hit points, and he is defeated when all forms have dropped to 0 hit points. See also death change.

ACTIONS

Multiattack. Virizan can use its Frightful Presence. He then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +15 to hit, reach 15 ft., one target.
Hit: 19 (2d10 + 8) piercing damage plus 10 (3d6) poison damage.

Claw. Melee Weapon Attack: +15 to hit, reach 10 ft., one target.
Hit: 22 (4d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +15 to hit, reach 20 ft., one target.
Hit: 17 (2d8 + 8) bludgeoning damage.

Frightful Presence. Each creature of Virizan’s choice that is within 120 feet of him and aware of him must succeed on a DC 19 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the Virizan’s Frightful Presence for the next 24 hours.

Caustic Breath (Recharge 5–6). Virizan exhales poisonous gas in a 90-foot cone. Each creature in that area must make a DC 22 Constitution saving throw, taking 77 (22d6) poison damage on a failed save, or half as much damage on a successful one.

REACTIONS
Death Change.When reduced to 0 hit points, Virizan uses exarch’s shape, and the new form takesdamage equal to half the damage of the effect that reduced the previous form to 0 hit points.

LEGENDARY ACTIONS

Virizan 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. Virizan regains spent legendary actions at the start of its turn.

• Detect. Virizan makes a Wisdom (Perception) check.
• Action. Virizan uses an Action option of the current form (but not multiattack).
• Fury (Costs 2 Actions). Virizan uses the current form’s multiattack.

NAGA
Virizan turns into a guardian naga with maximum hit points adding the Exarch’s Thunder attack option to the naga’s list of Actions:  

Exarch’s Thunder (Recharge 5–6). Virizan roars in a 90­-foot cone. Each creature in that area must make a DC 22 Constitution saving throw, taking 77 (22d6) thunder damage on a failed save, or half as much damage on a successful one.  Creatures that fail the save are pushed back 30 feet.

SNAKE SWARM
Virizan turns into a gargntuan swarm of poisonous snakes with 170 hit points adding the Swarm Attack trait swarms’ list of traits:

Swarm Attack. Each enemy that starts in the same space as Virizan’s swarm automatically takes bite damage from the swarm. The target is also distracted and has disadvantage on all attack, saving throws, and ability checks until it leaves the swarms space.

SNAKETONGUE
Virizan turns into a human archmage with 170 hit points adding the following Actions to the archamage’s standard list:

Mindspike (Recharge 5–6). As a bonus action Virizan magically emits psychic energy in a 30-foot cone. Each creature in that area must succeed on a DC 15 Intelligence saving throw or take 23 (4d8 + 5) psychic damage and ongoing 5 psychic damage per round for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Overwhelm Mind (Recharges when Mindspike does).  As a bonus action or reaction Virizan targets a creature taking ongoing psychic damage.  That creature must make a DC 15 Wisdom saving throw or become charmed by Virizan.

Emerald Coils. As a bonus action, Virizan points his serpent staff at two creatures he can see.  They each must make a DC 14 Dexterity saving throw or take 2d6 + 7 poison damage, and be pushed or pulled up to 15 feet (Virizan’s choice).

And replace the archmage’s dagger attack with a serpent staff attack:

Serpent Staff. Melee Weapon Attack: +6 to hit, reach 5 ft., one target.
Hit: 5 (1d6 + 2) bludegoning damage and 20 (3d10+5) poison damage.