Vaal Hazak, the Undying King

Eternal Bond. Vaal Hazak has bonded himself to you. Whenever you receive healing from your abilities, He is healed as well. 

Disperse. Release the animation on the bone and flesh holding you together to escape into the earth. 

Emerge. As long as there are bones and decaying nearby, the Death Knight can always reanimate Vaal Hazak to his side. (See Summon Death Drake for more details.)

 Undying Side Effects. Similar to the Death Knight, being undead comes with some benefits and drawbacks. 

• You take 1d10 bonus damage whenever you are hit with a radiant attack or effect. This includes sources of healing such as cure wounds. 
• However, if you are hit with any necrotic damage sources such as the blight spell, you are healed for that amount instead of taking damage. 
• You also do not need to sleep, eat or breath. You cannot suffocate or starve, but can be put unconscious by the effects of a spell or life-threatening damage. 

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage.

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d6 + 10) slashing damage.

Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 21 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 dragon's Frightful Presence for the next 24 hours.

• Dominate Undead. Any undead creature that can hear or feel the terrifying baleful cries, roars, screams and/or moans of Vaal Hazak must make a DC 25 Charisma save or be charmed/dominated by Vaal Hazak. 

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

Cold Breath. The dragon exhales an icy blast in a 90-foot cone. Each creature in that area must make a DC 24 Constitution saving throw, taking 67 (15d8) cold damage on a failed save, or half as much damage on a successful one.

Paralyzing Breath. The dragon exhales paralyzing gas in a 90-foot cone. Each creature in that area must succeed on a DC 24 Constitution saving throw or be paralyzed 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.