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 18, +11 to hit with spell attacks). viserion has following warlock spells prepared:
Cantrips (at will): blood bolt, eldritch blast, fell fire bolt, fire claws
1st level (5 slots): hellish rebuke, charm person, dessonant whispers

2nd level (4 slots): blood bath, ray of enfeeblement, crown of madness 

3rd level (4 slots): bone spear, bone trap, summon lesser demons

4th level (3 slots): darkfire, blight, summon greater demon

5th level (3 slots): control lava, hold monster, negative energy flood

6th level (2 slots): circle of death, investiture of flames

7th level (2 slots): finger of death, crown of stars (deals fire damage instead)

8th level (2 slots): aging, dominate monster

9th level (1 slot): into chaos

devilish commander: viserion can give a fiend any verbal command and the fiend has to obey to said command

legendary resistance (3/long rest): whenever viserion fails a saving throw he can choose to succeed instead

conjure black demons: every creature within a 15 foot area must make a dc 19 dexterity saving throw or take 20(4d10) piercing damage

blood curse:  Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 20 (2d12 + 10) necrotic damage. on a hit the target must make a wisdom saving throw or lose 1 point of constitution. when the target reaches 0 constitution it dies 

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

viserion has 3 legendary actions and regainst 2 at the start of each of his turns

spells from below: viserion casts a spell

blood touch (2 actions): viserion uses his blood curse action