Amphibious. The hag can breathe air and water.

Innate Spellcasting. The hag's innate spellcasting ability is Charisma (spell save DC 15). She can innately cast the following spells as an action, requiring no material components:

At will: poison spray, primal savagery, thorn whip

3/day each: charm person, command, faerie fire, ray of sickness 

2/day each: enlarge/reduce, mirror image, misty step

1/day each: fear, hypnotic pattern

Mimicry. The hag can mimic animal sounds and humanoid voices. A creature that hears the sounds can tell they are imitations with a successful DC 15 Wisdom (Insight) check.

Fey Movement. The swamp hag is immune to the effects of difficult terrain while in her lair.

Claws. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 1) slashing damage.

Invisible Passage. The hag magically turns invisible until she attacks or casts a spell, or until her concentration ends (as if concentrating on a spell). While invisible, she leaves no physical evidence of her passage, so she can be tracked only by magic. Any equipment she wears or carries is invisible with her.

Multiattack Whenever the hag attacks with its claws, it makes two attacks.

Drowning Grasp: When a creature is grappled by the Swamp Hag's Drowning Grasp attack, she may use her bonus action at the start of her turn to command the spectral hands to crush the target in their grasp. The hands make an attack roll with advantage on a grappled creature, dealing 14 (4d6) necrotic damage on a hit.

The swamp hag can take 1 legendary action, 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. The swamp hag regains spent legendary actions at the start of its turn.

Cackle. The hag targets one creature within 30 feet of it. If the target can hear the hag, the target must succeed on a DC 15 Charisma saving throw. On a failed save, the target has disadvantage on ability checks, attack rolls, and saving throws until the start of the hag's next turn.

Drowning Grasp. Ranged Attack: Range 60 ft., one target. The Swamp Hag reaches out with ghostly, watery hands and attempts to drown a creature in a mire of despair. The target must make a DC 15 Strength. On a failed save, the target is grappled by the spectral hands, and the Swamp Hag can choose to pull the target 10 feet closer to her. If a target is grappled by this attack, the Swamp Hag may use her bonus action to attack the target again. The hands make an attack roll with advantage on a grappled creature, dealing 14 (4d6) necrotic damage on a hit. The grappled creature can use its action to repeat the save on its turn, otherwise the hand is not dispelled until the Hag takes its next legendary action.

Wicked Claw.  Melee Spell Attack: +7 to hit, reach 5 ft., one target. Hit: 16 (3d10) necrotic damage.