Help with BBEG

[GrinningDemon]

I'm trying to build my end game BBEG and I'm looking for suggestions to craft a truly epic final boss.
The boss is a demonic immortal jester who has absorbed the souls of a demon prince, a level 20 paladin champion corrupted into an Oath breaker, an eternal undead giant king, a dead god and thousands of mortal souls. He changes into a primal being set to destroy the world. The mini I going to use is below, I'm struggling with coming up with abilities, spells, and health. I was thinking of just cherry picking from the abilities of devoured but I don't know for sure. I don't want to make it impossible for them to win but I would like it to be very challenging.

https://cults3d.com/en/3d-model/game/the-broken-one

[JNAProductions]

That's a cool mini.
What level do you anticipate your PCs being when they face this boss?

[GrinningDemon]

Between 18-20 probably closer to 20?

[JNAProductions]

Basic Numbers
Imma aim for CR 24-26 here.
Effective HP should be in the 536-670 range.
AC base is 19.
Attack bonus base is +12.
200-250 DPR.
Save DC base is 21, but this feels like an attack bonus monster more than a save DC one.

Note
A solo monster is a recipe for disaster. But this guy, from what you posted, doesn't feel like they'd have trusted advisors. So, Imma do a summoning/splitting the self kinda thing.

The Jester
Large Fiend, Chaotic Evil
Armor Class 21 (Cloak Of Souls)
Hit Points 600 (48d10+336)
Speed 50', Fly 30'


Strength
Dexterity
Constitution
Intelligence
Wisdom
Charisma


28 (+9)
24 (+7)
24 (+7)
20 (+5)
12 (+1)
30 (+10)


Skills Athletics +17, Acrobatics +23, Sleight of Hand +23, Stealth +15, Arcana +13, History +13, Religion +13, Deception +18, Performance +26
Damage Resistances Cold, Fire, Lightning, Thunder
Damage Immunities Poison
Condition Immunities Charmed, Frightened, Poisoned
Senses Darkvision 120', Passive Perception 11
Languages Many
Challenge Somewhere in the 24-26 range
Cloak Of Souls
The Jester is wreathed in the souls of those it has consumed. In addition to improving its AC to 14+Dexterity modifier, it begins each combat with 8 Soul Charges, used in various abilities. There is no maximum to the amount of Soul Charges the Cloak can hold. The Cloak regains a charge under the following conditions:
-A suitable creature is dropped to 0 HP or dies within 120'. A creature is suitable if they are CR 2 or greater-simply killing Joe Schmoe Ordinary won't do anything.
-A powerful creature is bloodied (dropped to half HP) for the first time in a fight within 120' of The Jester. A creature is considered powerful if they are CR 6 or greater.
-A worthy foe takes more than 40 damage in a single instance while within 120'. A worthy foe is a creature of CR 15 or greater.
Note that The Jester qualifies as a suitable and powerful creature, as well as a worthy foe.

Finally, the Cloak carries a retaliatory aura around it. Any creature that damages The Jester while within 120' takes 5 psychic damage. This increases to 10 psychic damage within 60', and 20 within 30'.

Aura Of Desolation
All allied creatures within 30' of The Jester deal an extra 10 necrotic damage on their attacks, provided The Jester is conscious. (This is NOT already included in The Jester's statblock.) Additionally, whenever a hostile creature within the aura takes necrotic damage, they must make a DC 18 Constitution save, or have their maximum HP reduced by the same amount as the necrotic damage they took. The maximum HP returns after completing a long rest.

Giant's Build, Jester's Frame
The Jester may count themselves as large as gargantuan or as small as tiny, including both simultaneously.

Actions
Unarmed Strike Melee Weapon Attack
+17 to-hit, reach 15', one target. Hit: 4d12+9 bludgeoning damage.

A Cruel Jest Ranged Spell Attack
+18 to-hit, range 120', one sentient target. Hit: 3d10+10 psychic damage, and the target is inflicted with a random malady from below if they fail a DC 18 Charisma save:
1) Cacklerot: The target laughs uncontrollably. Their move speed is halved, and they cannot cast any spell requiring a verbal component.
2) Festering Anger: The target grows uncontrollably angry and reckless. They gain advantage on all weapon attacks they make, but all attacks against them gain advantage.
3) Blissful Ignorance: The target suffers disadvantage on any checks, saves, or attacks using Intelligence, Wisdom, or Charisma.
A creature can be affected by more than one malady at the same time. At the end of each of their turns, they may repeat the Charisma save to shake off all maladies currently afflicting them. A Lesser Restoration allows for an immediate new save with advantage, while Greater Restoration or similar powerful abilities immediately heal all current maladies.

Extra Attack III
The Jester makes up to four attacks, choosing from the above actions. Up to two attacks may be replaced by Dash, Disengage, or Hide; while up to one attack may be replaced by a single action from below. Any attacks may be replaced by things that normally replace attacks, like an attempt to Shove or Grapple.

Revision (Three Soul Charges)
The Jester begins to bend reality to its pleasing. For the next minute, as long as it maintains concentration (as if on a spell) whenever a d20 is rolled involving it, roll one additional d20 and discard one of The Jester's choice.

Cavort In Agony (One Soul Charge)
The Jester creates a horrifying dance, that pains those who look at it. Anyone who can see The Jester when it starts the dance must make a DC 18 Wisdom save or suffer 2d10 psychic damage. Furthermore, until the end of The Jester's next turn, any attacks made against it using an ability score other than Strength suffer disadvantage. The Jester may extend this effect for another turn by using 30' of its move speed to dance.

Split Psyche (Two Soul Charges)
The Jester summons a Giggling Minion within 30' of itself.

Bonus Actions
Quick Action
The Jester makes an attack, Dashes, Disengages, or Hides.

Imitation
The Jester casts a spell that it saw cast between the end of its last turn and now.

Reactions
Twist
In response to being targeted for an attack, The Jester imposes disadvantage on the roll.

Roll With The Punches
The Jester halves damage from a single instance of damage.

Legendary Actions
The Jester can take five 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 Jester regains spent legendary actions at the start of their turn.

Attack. The Jester makes an Unarmed Strike or A Cruel Jest attack.
Split Psyche (Two Actions). The Jester uses the Split Psyche action.
Regain Reaction (Two Actions). The Jester regains its reaction.
Imitation (Three Actions). The Jester uses Imitation.

Giggling Minion
Medium Aberration, Chaotic Evil
Armor Class 16
Hit Points 190 (20d8+100)
Speed 40'


Strength
Dexterity
Constitution
Intelligence
Wisdom
Charisma


18 (+4)
22 (+6)
20 (+5)
10 (0)
6 (-2)
16 (+3)


Skills Athletics +8, Acrobatics +14
Damage Immunities Psychic
Senses Darkvision 60', Passive Perception 8
Languages Laughter
Challenge 8
Hive Mind
All Giggling Minions are salved to The Jester. Whatever they know, it knows. What they see, it sees. What they hear, it hears.

Actions
Unarmed Strike Melee Weapon Attack
+10 to-hit, reach 10', one target. Hit: 2d8+6 bludgeoning damage.

Lance Of Laughter Ranged Spell Attack
+7 to-hit, range 90', one target. Hit: 2d6+3 psychic damage.

Extra Attack II
The Giggling Minion makes up to three attacks from the actions above or other attack actions.

Bonus Actions
Cunning Action
The Giggling Minion uses the Dash, Disengage, or Hide action.

Reactions
Protect
If the Giggling Minion is between The Jester and an attacker, it can use its reaction to interpose itself and take the attack instead of its master.

Not TOTALLY satisfied with this, but it's a good start, at least.

[GrinningDemon]

Wow that is definitely bbeg level there. It looks amazing

[JNAProductions]

Wow that is definitely bbeg level there. It looks amazing

Thank you! Feel free to use it, tweak it, whatever works for you. And do let me know how the game goes. :)

[GrinningDemon]

I will diffidently give an update on how the battle goes, they still have 3 more major battles ahead so it will probably be a month or two before they find this beauty. Thank you so much for the help with this, I will update you with any twekes or changes I make to it.

[Yakk]

What I'd be tempted to do with this is to have the PCs face the fragments of its soul separately.

demon prince
a level 20 paladin champion corrupted into an Oath breaker
an eternal undead giant king
a dead god

Then the final BBEG fight would be them merged together.

Some of the initial fights are against partial parties.

This also lets you calibrate how badass the party is.

For the Oath breaker, a Death Knight against 1 or 2 PCs. Maybe with the rest of the party fighting one of the other aspects. And after they beat their half, they can come rescue the other team.