Archeoness Fiadhnan Cait of the Tríú

Fey Pureblood. Iron weapons count as magical weapons for the purpose of determining damage resistance.

Magic Resistance. Has advantage on all saves made against spells and magical effects. 

True Fey Ancestry: Immune to being Charmed, and magic can't put them to sleep. 

Legendary Resistance (3/Day). If they fail a saving throw, they choose to succeed instead.

Shrouded In Shadows. The shadows surround them. When in Darkness they become completely invisible. 

Destroy Light (3/Day). With a snap of their fingers all light within a 120 range is snuffed out for (1d6) rounds.

Wild Walker. This creature can pass freely between the Prime Material Plane and the Feywild unless restricted with Iron or magical means.

Spellcasting. Archeoness Fiadhnan Cait of the Tríú is a 18th level spellcaster. Her spellcasting ability is charisma (spell save DC 18, +6 to hit with spell attacks). She has the following spells prepared:

Cantrips (at will): Friends, Prestidigitation, Thaumaturgy

1st level (3 slots): Charm Person, Command, Cure Wounds, Faerie Fire, False Life, Healing Word, Sleep

2nd level (3 slots): Detect Thoughts, Gust of Wind, Healing Spirit, Hold Person, Lesser Restoration, Suggestion

3rd level (2 slots): Counterspell, Dispel Magic, Revivify, Sleet Storm, Slow

4th level (2 slots): Charm Monster, Confusion, Dimension Door, Dominate Beast, Fabricate

5th level (3 slots): Dominate Person, Geas, Greater Restoration, Hold Monster, Telekinesis

 6th level (1 slots): Conjure Fey

7th level (1 slots): Regenerate

8th level (1 slots): Feeblemind

Fey Cait Sidhe. In a swirl of smoke and shadows Fiadhnan Cait can switch between her elven appearance and that of a panther with a white star on her chest. This form has its own attacks, a +10 to dex, and +5 to strength when used to grapple. 

Claws (Cait form). Melee Weapon Attack: +10 to hit, reach 10 ft, 1 target: Hit: 20 (2d10+10)

Bite (Cait form). Melee Weapon Attack: +8 to hit, reach 5 ft., 1 target. Hit:  15 (5d6 + 5) piercing damage.

Pounce (Cait form). Fiadhnan Cait attempts a grapple on a creature within 10 feet. If successful, she immediately makes a bite attack to follow. 

Predatory Gaze (Both Cait form and Sidhe form). Fiadhnan Cait locks eyes with a living creature. The creature must make a wisdom saving throw vs Fiadhnan Cait's intimidation, or become frightened.

Cat Like Reflexes (Cait Form). In reaction to melee attack or an attack made within ten feet, the creature must make a dexterity save vs Fiadhnan Cait's acrobatics or be knocked prone as she jumps to a square within 10 feet. As long as the attack beats Fiadhnan Cait's AC, it does not fail, even if the creature fails their dexterity save.

Being a true fey queen from the first era, the Fiadhnan Cait has 3 legendary actions. She may use one at the end of another creature's turn. She regains her legendary actions at the start of her own turn.

Relentless Stalker (Costs 1 Actions). When a creature moves on their turn, the Fiadhnan Cait can move up to her movement up to them.

Cait Form/Sidhe Form. (Costs 1 Actions). On another creature's turn Fiadhnan Cait can switch between her forms.

Lesser Spellcasting (Costs 2 Actions). Fiadhnan Cait casts a 3rd level spell or lower.

Apex Predator (Costs 3 Actions). Fiadhnan Cait makes a pounce attack on a creature within reach.