Name: Muilearteach, the Sea Mither
Large fey, chaotic neutral
Armor Class: 16 (natural armor)
Hit Points: 200 (16d10 + 100)
Speed: 30 ft., swim 60 ft.

STR: 20 (+5)
DEX: 16 (+3)
CON: 20 (+5)
INT: 16 (+3)
WIS: 20 (+5)
CHA: 18 (+4)

Skills: Perception +10, Medicine +10, Nature +10
Damage Resistances: Cold, Thunder
Senses: darkvision 120 ft., passive Perception 20
Languages: Common, Sylvan, Aquan
Challenge Rating: 18 (20,000 XP)

        Innate Spellcasting: The Muilearteach's spellcasting ability is Wisdom (spell save DC 20). The Muilearteach can innately cast the following spells, requiring no material components:

        At will: control water, fog cloud, shape water, water breathing
        3/day each: call lightning, heal, tidal wave
        1/day: control weather, tsunami

        Legendary Resistance (3/day): If the Muilearteach fails a saving throw, it can choose to succeed instead.

Actions:
        Multiattack: The Muilearteach can use its Frightening Presence. It then makes three attacks: two with its claws and one with its bite.
        Claw: Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 15 (2d10 + 4) slashing damage.
        Bite: Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 17 (2d12 + 4) piercing damage.
        Frightening Presence: Each creature of the Muilearteach's choice within 30 feet of it and aware of it must succeed on a DC 18 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, 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 Muilearteach's Frightening Presence for the next 24 hours.

Legendary Actions:
        The Muilearteach 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 Muilearteach regains spent legendary actions at the start of its turn.
        Claw Attack: The Muilearteach makes one claw attack.
        Call Lightning (Costs 2 Actions): The Muilearteach casts call lightning.
        Shape Water (Costs 1 Action): The Muilearteach uses its Shape Water spell-like ability.

Description: The Muilearteach, also known as the "Sea Mither" or "Storm Mother", is a mythical creature from Scottish folklore that is said to reside in the depths of the ocean.

According to legend, the Muilearteach appears as a giant, hag-like woman with wild, tangled hair and a long, flowing gown made of seaweed and shells. Her skin is said to be a pale, sickly green color, and her eyes are described as glowing with an eerie light that can be seen from a great distance in the darkness of the stormy sea.

As the "Sea Mither," the Muilearteach is believed to have the power to control the elements of the ocean, and is said to be able to summon up great storms and rough seas. She is also said to have the ability to heal sick or injured sailors and to guide lost ships safely to shore.

Despite her fearsome appearance and reputation, the Muilearteach is often seen as a protector and guardian of those who make their living on the sea, and is respected and revered by many in Scottish coastal communities.