Algrick the Swamp Hag

Disguise Self. Algrick can alter her appearance as an action to look like any Medium humanoid she has seen before.

Swamp Mastery. Algrick has advantage on Dexterity (Stealth) checks made to hide in swampy terrain.

Legendary Resistance (3/Day). If Algrick fails a saving throw, she can choose to succeed instead. 

Spellcasting. Algrick is a 15th-level spellcaster. Her spellcasting ability is Intelligence (spell save DC 16, +8 to hit with spell attacks). Algrick has the following spells prepared:

Cantrips (at will): Mage Hand, Minor Illusion, Prestidigitation

• 1st level (4 slots): Charm Person, Fog Cloud, Sleep
• 2nd level (3 slots): Detect Thoughts, Hold Person, Mirror Image
• 3rd level (3 slots): Hypnotic Pattern, Stinking Cloud, Vampiric Touch
• 4th level (3 slots): Charm Monster, Dimension Door, Polymorph
• 5th level (2 slots): Dominate Person, Insect Plague, Telekinesis
• 6th level (1 slot): Mass Suggestion, True Seeing
• 7th level (1 slot): Etherealness, Reverse Gravity
• 8th level (1 slot): Dominate Monster, Earthquake
• 9th level (1 slot): Shapechange

Multiattack. Algrick makes two attacks with Foul Slash and one attack with Swamp Venom Spray.

Foul Slash. Algrick makes a melee attack with a +12 to hit. On a hit, the target takes 31 (6d8 + 4) slashing damage.

Swamp Venom Spray. Algrick makes a ranged attack with a +12 to hit. On a hit, the target takes 42 (8d8 + 6) poison damage. The target must then make a DC 19 Constitution saving throw or become poisoned.

Swamp Vortex. Algrick creates a swirling vortex of swampy energy in a 30-foot radius around her. Each creature in that area must make a DC 19 Dexterity saving throw. On a failed save, the creature takes 82 (15d10) necrotic damage. On a successful save, the creature takes half as much damage.

The monster 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. The monster regains spent legendary actions at the start of its turn.

Algrick may only use her Mucky Grasp Legendary Action if she is in conditions that are wet or swamp-like. All other actions may be taken as described above.

Mucky Grasp (Costs 2 Actions). Algrick reaches out with her slimy, muck-covered hand to grasp the target. She makes a melee attack with a +12 to hit. On a hit, the target takes 27 (5d8 + 4) bludgeoning damage and is restrained until the start of Algrick's next turn.

Misty Escape (Costs 1 Action). Algrick envelops herself in a shroud of mist and disappears, reappearing up to 30 feet away. This movement doesn't provoke opportunity attacks.

Swamp's Curse (Costs 3 Actions). Algrick casts a powerful curse on a target within 60 feet. The target must make a DC 19 Wisdom saving throw. On a failed save, the target becomes cursed and suffers disadvantage on all attacks and saving throws until the curse is removed. Additionally, the target takes 39 (7d10) psychic damage at the start of each of its turns as the curse gnaws away at its mind.