Innate Spellcasting: Yu'lulth's innate spellcasting ability is Charisma. It can innately cast the following spells (spells save DC 15)
At will: [Tooltip Not Found] , Mage armor* 
1/day each: Scrying , Arcane gate 

Spellcasting: Yu'lulth'sis a 14th-level Spellcaster. His Spell casting ability is Wisdom (spell save DC 17, +9 to hit). He regains its expended spell slots when it finishes a short or long rest.
He knows the following spells:
Cantrips: Eldritch blast , Guidance , Mage hand , [Tooltip Not Found] .

1-5th level (4 5th-level slots):
Arms of Hadar 
Crown of madness 
Contact other plane 
Dimension door 
Dissonant whispers 
Dominate Person 
Vampiric Touch 
Hunger of Hadar 
Darkness 
[Tooltip Not Found] 

Drowned Mind: This creature has advantage on Wisdom Saving Throws

Hungering One's Deep: After 1 minute of chanelling a shadowy domain appears generating from the user. After the channeling stops 4 5ft radius pool of shadowy black tentrils appear in a point that the user can see. The selected point for each one is different and they are within a 100ft radius from the caster. These pools have they're own initiative (half of that of the user). They're also considered difficult terrain for all other creatures except the user. On their turn they make two Tentacle attacks to creatures inside the pools.

*The archmage casts these spells on itself before combat.

 Actions: Yu'lulth either does attack with his tentacles or a multiattack, one with his claw and one with his bite

Tentacles. Melee Weapon Attack: +8 to hit, one target. Hit: (2d6+2) slashing damage

Claw. Melee Weapon Attack: +5 to hit, reach 5ft., one target. Hit: (1d8+2) slashing damage

Bite. Melee Weapon Attack: +5 to hit, reach 5ft., one target. Hit: (1d6+2) piercing damage The user regains (1d10+2) hit points.

Enter the description for how bonus actions work for your monster here.

Wavering Shield: After Hungering One's Deep has been casted as a reaction the user can "consume" 2 of the pools, in order to protect himself. This gives a bonus +4 AC till the end of turn. .

This creatures legendary actions can only be accessed if the Hungering One's Deep has been casted. This creature can take 2 legendary actions per turn cycle.Legendary Action Name (Costs # Actions). 

Shifting Shadows: He can move the Hungering One's Deep pools up to 15 ft.

Binding: The user of Hungering One's Deep pools can make up to 1 of the pools to grapple a target (Strength DC 17)

Engorging Darkness (2 actions): The user summons 1d4-2 (with a minimum of 1) Hungering One's Deep pools.

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Mythic Action Name (Costs # Actions). Enter the description for your mythic action.