Host's Form. As a result of her being in a borrowed body, her host gains their own initiative based on their stats (assume they are level 20 as well). Each turn they may make a save of the DM's choice,  depending on how the player is planning on trying to regain control. If they succeed a DC 12 save they can make Zeboim roll disadvantage on one of the following:

• An effect saving throw
• An attack against their ally
• Destroying an object/ building they care for.

The DC increases by 2 each turn they continue to be in combat.

Spellcasting. Zeboim is a 20-level spellcaster. Her spellcasting ability is Wisdom (spell save DC #, +# to hit with spell attacks). Zeboim has the following Druid spells prepared: 

• Cantrips (at will): Mending, Poison Spray, Produce Flame, Shillelagh
• 1st level (4): Create or Destroy Water, Animal Friendship, Charm Person, Faerie Fire, Thunderwave
• 2nd level (4): Barkskin, Enhance Ability, Flaming Sphere, Gust of Wind, Hold Person, Lesser Restoration, Moonbeam
• 3rd level (4): Call Lightning, Dispel Magic, Sleet Storm, Water Breathing, Water Walk
• 4th level (3): Blight, Conjure Minor Elementals, Control Water, Dominate Beast, Ice Storm, Polymorph
• 5th level (2): Antilife Shell, Conjure Elemental, Geas, Planar Binding 
• 6th level (2): Conjure Fey, Wind Walk
• 7th level (1): Plane Shift, Reverse Gravity
• 8th level (1): Control Water, Feeblemind
• 9th level (1): Shapechange, Storm of Vengeance, True Ressurection

Multiattack. Zeboim can use her Tsunami ability if it is available, and may make three attacks afterward: one water tendril attack and two thunderbolt strikes

Water Tendrils. Melee Weapon Attack: +11 to hit, reach 40 ft., 3 targets. Hit: 16 (4d6 + 2) bludgeoning damage, plus 16 (3d10) Cold. Opposed Athletics to grapple one person per turn, DC 15.

Thunderbolt Strike. RangedWeapon Attack: +22 to hit, range 20/160 ft., 1 target. Hit: 27 (4d6 + 13) lightning damage.

Tsunami (5/6). Each Creature within a 100' radius of Zeboim must make a DC 30 Strength save of they are swept into her storm and cannot make an action against her this turn. On a fail, they take 11 (2d10) bludgeoning damage and 11 (2d10) cold damage. On a success, they take half as much damage and are able to navigate her storm without negative effects.

Presence. Zeboim may choose one of three presence options. She may not use the same option more than once per turn.

• Frightful Presence. Each creature of Zeboim's choice within 120' of her must make a DC 27 Wisdom save of they will become frightened for one minute. A creature can repeat their saving throw at the end of its turn with a disadvantage if Zeboim is in their line of sight. If their saving throw is successful or the effect ends for it, the creature becomes immune to Zeboim's Frightful Presence for the next 24 hours.
• Maddening Presence. Each creature within Zeboim's choice within 120' of her must make a DC 27 Wisdom saving throw or will suffer from one of the following maddening effects:

1. Your mind is drained and sluggish. You gain one level of exhaustion and do not gain the benefit of a short rest. 
2. Your sensations are muddled, and you seem to hear colors and taste sounds. You have disadvantage on perception rolls and your passive perception becomes 0.
3.  You find yourself compelled to eat small inedible objects. You must make a DC 10 Charisma save to resist consuming these objects at the beginning of each turn. Each object you consume does 1d4 damage to you.
4.  The last few hours are a blur to you, you lose all effects that you've gained since your last long rest, and forget everything that has happened since then.
5.  You find yourself easily distracted and unable to focus on any single task or target. You cannot choose to repeat the same action or target the same object/creature two times in a row. 
6.  A guilty conscience weighs on you heavily, holding you back from deceptive or immoral acts. You now have disadvantage on all skill checks to lie, deceive, or steal.
7.  You develop a noticeable vocal tick, which others find unsettling. You have disadvantage on deception, persuasion, and performance checks that rely on speech.
8.  The idea of forcing your will onto others becomes frightening and difficult for you. You have disadvantage on persuasion and intimidation checks. 
9.  Your fingers now struggle to remember the nuances of written language. You must succeed on a DC 10 Dexterity save in order to write legibly.
10.  You find yourself pocketing anything not nailed down. If you are unobserved within 5' of a small object that would fit into the small of your hand, you must succeed on a DC 15 Charisma check, or you attempt to pocket the item.
11.  Written language becomes difficult as the symbols on the page seem to dance around. You must succeed on a DC 10 Intelligence check to read any writing, even if you are able to normally read that language.
12.  Your tongue ties itself into knots whenever you attempt to speak. You must succeed at DC 10 Dexterity check in order to speak coherently.
13.  The difference between dreams and memory is ill-defined. You have disadvantage on knowledge checks made to recall information, however, on a natural 1, you recall irrelevant information that you have had no way of knowing.
14.  You find it bewildering to face asymmetry in battle. When there are an odd number of enemies in combat, you have disadvantage on attack rolls.
15.  A vast hunger consumes you. You must succeed on a DC 15 Charisma save to avoid ingesting every food and drink provided to you, and you suffer 1 point of exhaustion for every hour you go without food or drink.
16.  You find it increasingly difficult to tell friend from foe. You must succeed on a DC 10 Charisma check each turn to correctly identify a creature as an enemy or ally in combat.
17.  It becomes difficult to tell the truth, even when you know you should. You add twice your proficiency to deception checks when telling a lie but must succeed on a DC 15 Charisma save, to tell the truth.
18. Your dreams are plagued by terrible nightmares. You must succeed on a DC 15 Charisma check to gain the benefits of any long rest you take.
19.  Pain is so deadened by your mind that you have difficulty recognizing when to avoid it. You gain temp hp equal to your Charisma modifier each round, but you must subtract your Dexterity modifier from your AC
20.   You are Uncharicteristically chipper, and gain advantage on all skill checks with which you are proficient. People become naturally suspicious of you.

A creature may repeat their save at the end of their turn, ending all effects of Maddening Presence on a success.

• Suffocating Presence. Each creature of Zeboim's choice within 120' of her must make a DC 27 Wisdom saving throw. On a failure, they take 16 (3d10) cold damage or half damage on a success. Additionally, Zeboim becomes surrounded by a 60' radius of suffocating fog that reduces all light to dim light and creates difficult terrain. Any creature within the fog cannot breathe or speak. Any creature beginning their turn in the fog takes 1d6 damage. The fog recedes and the effect ends at the beginning of Zeboim's next turn.

Legendary Resistance (3/Day). When Zeboim fails a saving throw, she may choose to succeed instead.

Siren's Call. Zeboim may summon 1d6 minor water/storm creatures each turn she's near water as a bonus action.

Titan's Blessing: One time per host Zeboim may summon the Kraken to her aid. She commands the will of the monster, as long as she is above 1/2 health. Once she loses control; it may choose to leave the battle, continue to attack, or attack her depending on how the battle is going. [It is unlikely the last will happen, as she is its caretaker.]

Storm's Fury: When a creature starts a turn within 20' of her in battle or hits her with a melee attack they must make a DC 27 Wisdom throw or take 2d10 Lightning Damage and 2d10 Psychic damage, or half damage on a success.

Zeboim can take 3 Legendary Actions. Only one option can be used at a time. She regains spent legendary actions at the start of her turn.

Windstorm Wail. Everyone within 600' must make a DC 27 Wisdom save. On a fail, they take 11(2d10), psychic damage and 5 (1d10) thunder damage. Creatures within 40' of her take an additional 16 (3d10) thunder damage.

Drain. Her host makes a DC 12 Wisdom save. On a fail, they are drained for 2d6 health, and Zebiom regains it. The DC save increases by 2 each time this is used between long rests. In the event that the host hits 0 hp (and fails 3 death saves on their turns), Zeboim is now free to do what she wants in her own body. Her size increases to Gargantuan.

Command (Costs 2 Actions). Zeboim may use one of her Presence options.