Crawling King

Dragonmillipede's Gas. The Crawling King has a 10ft. radius poisonous aura, and any creature that enters its range for the first time or ends its turn in it makes a DC 20 con save or takes 22 (4d10) poison damage and is poisoned or half as much on a successful save without any extra effects. 

Spellcasting. The Crawling King is a 20-level spellcaster. Its spellcasting ability is Charisma (spell save DC 20, +12 to hit with spell attacks). The Crawling King has following spells prepared and can cast them without any material components or the need to concentrate: 

Spells (at will): Insect Plague,  Giant Insect, Fireball

Assassin's Bug Shapeshifting. The Crawling King can make himself look, smell and sound like anything he has killed before. A creature musst make a DC 18 investigation check to discern that there is an illusion.

Spanish Fly's Poison. Any creature that makes a melee attack and hits the Crawling King must make a DC 20 con save or take 4d10 poison damage, be poisoned and be vulnerable to all damage until the poison wears off in an hour, or take half the damage and suffer no further consequences.

Cockroach's Undying. If the Crawling King is dropped to 0 hp, it will continue fighting and remain conscious. While in this state, the Crawling King has advantage on all saving throws and attack rolls and all actions recharge.

Flea's Jump. The Crawling King can jump 1000 feet. 

King of Worm. Insects gather around him and will fight for him

Blinding Speed. The Crawling King doesn't trigger opportunity attacks when moving.

Compound Eyes. The Crawling King has 360° vision and cannot be surprised.

Multiattack. The Crawling King makes four punch attacks on his turn. He can replace any punch attack with a crush attack.

Punch Attack. Melee Weapon Attack: +16 to hit, reach 5 ft., 1 target. Hit: 20 (3d6 + 10) bludgeoning damage. If the target is a medium creature, it must make a DC 22 strength save or be launched away 60 ft. from the Crawling King. For every size category underneath, the distance doubles (small=120 ft., tiny=240 ft.) and for every size above the distance halves (large=30 ft., huge=15 ft.)
Additionally, if the target collides with something else, it takes 1d10+5 damage for every 5 ft. that it had to travel, and whatever it collided with takes 1d10+5 damage for every 5 ft. that the target traveled.

Crush Attack. Melee Weapon Attack: +16 to hit, reach 5 ft., 1 target. Hit: 37 (5d10 + 10) force damage. If the target is a creature, it must succeed a DC 20 strength save or be grappled by the Crawling King. When the King is grappling a creature like this, he can no longer attack another target with that arm, and he has four. He can however, while grappling a creature, automatically deal the crush damage. If the Crawling King grapples a creature with all four of his arms, he can make a Dissecting Pull.

Cicada's Scream (recharge 5-6). The Crawling King breaths in and lets out a devastating scream that can be heard miles away. Every creature within 200 ft. must make a DC 18 wis saving throw or be frightened. If frightened a creature can make a saving throw at the end of its turn to end the effect.
Every creature within 60 ft. must make a DC 20 con save or be deafened for 1 minute, stunned for 1 round and take 3d10 thunder damage, or half as much damage on a success and suffer no further effects.
Every creature within 30 ft. must make a DC 20 con save or be knocked unconscious until someone wakes them up or they are attacked, and take 5d10 thunder damage, or half as much on a success and suffer no further consequences.
None of this works if the creature can't hear.

Dissecting Pull. If the Crawling King is grappling a creature with all four of his arms, then every crush attack on that target becomes a critical. If the creature dies from this, it's ripped into four parts.

Swarm Army. The Crawling King can cast Insect Plague.

Meat-shield. If the Crawling King is grappling one or more creatures, he can place them in between him and harm. If an enemy attacks the King with an attack requiring an attack role or targeting him with a line attack, he can place one target he is grappling in between and have it take the damage. The Crawling King can take this reaction as many times equal to the number of targets he is grappling.