Legendary resistance(3/Day): If viserion fails a saving throw, it can choose to succeed instead.

Magic resistance: viserion has advantage on saving throws against spells and other magical effects.

Spellcasting. viserion is a 20th-level spellcaster. Its spellcasting ability is charisma (spell save DC 20, +12 to hit with spell attacks). viserion has following warlock spells prepared: 
Cantrips (at will): eldritch blast, sword burst, corruption

5th (5 slots): control lava, dominate person, dance macabre, infernal calling

6th (4 slots): circle of death, investiture of flames, sould cage

7th (4 slots): finger of death, plane shift (self only), power word pain

8th (3 slots): aging, demiplane, power word stun 

9th (2 slots): black hole, a call beyond

infernal voice: viserion can say a verbal command and any creature with fiend blood must respond to said command

unholy healing: whenever viserion takes necrotic damage it instead heals him for the damage dealt. and if viserion ever consumes a small dose of blood he heals for 1d6 health

multiattack: viserion can make 3 attacks, two with its claws and one with its bite

claws. Melee Weapon Attack: +12 to hit, reach 5 ft., 1 target. Hit: 10(5d4) slashing damage and 10(5d4) necrotic damage

bite. Melee Weapon Attack: +12 to hit, reach 5 ft., 1 target. Hit: 13 (3d8 + 2) piercing damage and 8 (2d6 + 4) necrotic damage which heals viserion for said damage

blood slash: RangedWeapon Attack: +11 to hit, range 120 ft., 4 target. Hit: 12 (2d10 + 5) slashing damage.

chaotic magic: whenever viserion casts a spell it rolls a d100 and if it is a 10 or bellow he targets himself 11-50 targets the wrong target, 51-98 targets the intended target and 99-100 targets all enemies

infernal fireback: if viserion ever gets struck by a spell which deals necrotic or fire damage he can return the spell onto the caster