Amdusias

Hellish Weapons. The Amdusias' weapon attacks are magical and deal an extra 13 (3d8) thunder damage on a hit (included in the attacks).

Innate Spellcasting. The Amdusias' spellcasting ability is Charisma (spell save DC 18). The pit fiend can innately cast the following spells, requiring no material components:

At will: call lightning, thunder step 

3/day each: ice storm, storm sphere

1/day each: chain lightning, control weather, freezing sphere, whirlwind 

Magic Resistance. The Amdusias has advantage on saving throws against spells and other magical effects.

Multiattack. The Amdusias makes three attacks or two attacks and then use Storm Bringer 

Trident. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) piercing damage, or 9 (1d10 + 4) piercing damage if used with two hands, plus 13 (3d8) thunder damage. Each attack marks a creature with a storm mark which makes warm and cold air swarm around the target

Storm Bringer (Recharge 6).  When a creature is marked with a storm mark 3 times you can form a thundercloud around them. This cloud deals 40 (6d12) lightning damage and you become paralyzed for 1 minute. After using this ability you have  

Parry. The erinyes adds 4 to its AC against one melee attack that would hit it. To do so, the erinyes must see the attacker and be wielding a melee weapon.