Amphibious.

The hag can breathe air and water.

Horrific Appearance.

Any humanoid that starts its turn within 30 feet of the hag and can see the hag's true form must make a DC 11 Wisdom saving throw. On a failed save, the creature is frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the hag is within line of sight, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the hag's Horrific Appearance for the next 24 hours.

Unless the target is surprised or the revelation of the hag's true form is sudden, the target can avert its eyes and avoid making the initial saving throw. Until the start of its next turn, a creature that averts its eyes has disadvantage on attack rolls against the hag.

Actions

Claws.

Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) slashing damage.

Death Glare.

The hag targets one frightened creature she can see within 30 feet of her. If the target can see the hag, it must succeed on a DC 11 Wisdom saving throw against this magic or drop to 0 hit points.

Illusory Appearance.

The hag covers herself and anything she is wearing or carrying with a magical illusion that makes her look like an ugly creature of her general size and humanoid shape. The effect ends if the hag takes a bonus action to end it or if she dies.

The changes wrought by this effect fail to hold up to physical inspection. For example, the hag could appear to have no claws, but someone touching her hand might feel the claws. Otherwise, a creature must take an action to visually inspect the illusion and succeed on a DC 16 Intelligence (Investigation) check to discern that the hag is disguised.

Lair Actions

The following lair actions are options for grandmothers and powerful aunties. Grandmothers usually have three to five lair actions, aunties usually only one (if they have any at all). Unless otherwise noted, any lair action that requires a creature to make a saving throw uses the save DC of the hag's most powerful ability.

On initiative count 20 (losing initiative ties), the hag can take a lair action to cause one of the following effects, but can't use the same effect two rounds in a row:

Until initiative count 20 on the next round, the hag can pass through solid walls, doors, ceilings, and floors as if the surfaces weren't there.
The hag targets any number of doors and windows that she can see, causing each one to either open or close as she wishes. Closed doors can be magically locked (requiring a successful DC 20 Strength check to force open) until she chooses to make them unlocked, or until she uses this lair action again to open them.

A powerful sea hag might have the following additional lair actions:

The hag fills up to four 10-foot cubes of water with ink. The inky areas are heavily obscured for 1 minute, although a steady, strong underwater current disperses the ink on initiative count 10. The hag ignores the obscuring effect of the ink.
The hag chooses one humanoid within the lair and instantly creates a simulacrum of that creature (as if created with the simulacrum spell). This hideous simulacrum is formed out of seaweed, slime, half-eaten fish, and other garbage, but still generally resembles the creature it is imitating. This simulacrum obeys the hag's commands and is destroyed on initiative count 20 on the next round.

Regional Effects

Each hag's lair is the source of three to five regional effects; the home of a grandmother, an auntie, or a coven has more effects than the lair of a single hag, including some that can directly harm intruders. Any regional effect that requires a creature to make a saving throw uses the save DC of the hag's most powerful ability. These effects either end immediately if the hag dies or abandons the lair, or take up to 2d10 days to fade away.

The region within 1 mile of a grandmother hag's lair is warped by the creature's fell magic, which creates one or more of the following effects:

Birds, rodents, snakes, spiders, or toads (or some other creatures appropriate to the hag) are found in great profusion.
Beasts that have an Intelligence score of 2 or lower are charmed by the hag and directed to be aggressive toward intruders in the area.
Strange carved figurines, twig fetishes, or rag dolls magically appear in trees.

A powerful sea hag creates one or more of the following additional regional effects within 1 mile of her lair:

Most surfaces are covered by a thin film of slime, which is slick and sticks to anything that touches it.
Currents and tides are exceptionally strong and treacherous. Any ability check made to safely navigate or control a vessel moving through these waters has disadvantage.
Shores are littered with dead, rotting fish. The hag can sense when one of the fish is handled and cause it to speak with her voice.

Sea Hag