Fiendbinder. Fiends in service to Vechernyaya cannot act in a manner that would be contrary or a danger to Vechernyaya's interests unless directly commanded to do so by her.

Infernal Synergy. When Vechernyaya casts a spell she can cast a will each allied fiend within 60 feet of her can immediately take a reaction to also cast that spell, requiring no material components and using Vechernyaya's spell save DC and spell attack bonus. If there are no eligible targets for the spell a fiend casts in this manner, the effect fails. 

Innate Spellcasting. Vechernyaya's innate spellcasting ability is Charisma (spell save 22, +14 to hit with spell attacks). She can innately cast the following spells requiring no material components:

At will: burning hands, cure wounds (2nd level), fire bolt

3/day each: bestow curse, dispel magic, fireball, disguise self (including the form of a Medium Humanoid)

2/day each: fire shield, heal, magic circle

1/day each: power word heal

1/month: planar binding (9th level)

Multiattack. Vechernyaya casts two spells, one of which must be a spell she can cast at will.

Flame Whip. Melee Weapon Attack: +14 to hit, reach 60 ft., one target. Hit: 45 (10d8) fire damage.

Tortuous Empowerment (1/Short or Long Rest).Vechernyaya magically grants an allied fiend she can see within 60 feet of her the gift of overwhelming power for 1 minutes. While so empowered, the fiend is immune to the paralyzed, stunned and unconscious conditions, has advantage of attach rolls and saving throws, and can make an additional attach with it's Multiattack feature on each of it's turns. When the effect ends the fiend dies instantly and Vechernyaya can never again affect the fiend with a spell or magical effect.

Swap. As a reaction when Vechernyaya takes damage, she can magically swap places simultaneously with an allied fiend she can see within 60 feet of her, teleporting to each other's spaces or the nearest unoccupied space in the even either's size would prevent them from doing so.

Vechernyaya 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. Vechernyaya regains spent legendary actions at the start of her turn.

 Eat Grief. The Hag forces a creature she can see within 40 feet to make a DC 22 Wisdom saving throw as she pulls up a memory or fear. On a failed save the creature takes 4d6 psychic damage and the hag and any allied fiends heal for half that amount

Summon Fiends. The hag summons 6 CR1 fiends or 2 CR 5 fiends. Any CR 1 fiend functions as a minion with only 1 hitpoint. All other stats are the same. 

Cast Spell (Costs 2 actions). The hag casts an at will spell and uses infernal synergy.