Armagedadon

*ANY ADVICE ON HOW I CAN MAKE THIS THING COOL BUT NOT OP PLEASE LET ME KNOW, THIS IS INTENDED TO BE FOUGHT BY A LEVEL 17+ PARTY*

Arcane Acuity. The Armagedadon doesn't not require V, S, or M costs when casting spells

Master of the Sea. While the Armagedadon is in water, it has immunity to fire and cold damage

Spellcasting. The [monster name] is a #-level spellcaster. Its spellcasting ability is [ability score] (spell save DC #, +# to hit with spell attacks). The [monster name] has following [class name] spells prepared:
Cantrips (at will): [spell name], [spell name] 
1st level (# slots): [spell name], [spell name] 

Action Name. Enter the description for your action.

COMMAND: Chomp. Melee Weapon Attack: +9 to hit, reach 10 ft., 1 target. If the target is medium sized or smaller, it must make a DC 25 dexterity saving throw or become grappled and is stuck inside the Armagedadon's jaws until it decides to use  COMMAND: Swallow Creature or COMMAND: Chew, If the creature is not medium sized or smaller, take 43 (8d8+7) piercing magic damage

A victim trapped in the jaws can make a DC 15 Athletics check to be let go or the Armagedadon must take 30 damage and it will release its jaws

COMMAND: Swallow Creature. After the target of COMMAND: Chomp has been grappled for one round, the Armagedadon may attempt to swallow the creature, instantly killing it.

If the target is above 75% hp (rounded up), the Armagedadon must succeed a DC 28 survival check with disadvantage. If the target is above 50% hp (rounded up), the Armagedadon must succeed a DC 26 survival check. If the target is below 50% hp (rounded up), the Armagedadon must succeed a DC 23 survival check. If the target is below 25% hp (rounded up), the Armagedadon must succeed a DC 20 survival check with advantage.

COMMAND: Dread Cannon. Shoots ammo dictated by ORDER: Load Dread Cannon

COMMAND: Chew. After the target of COMMAND: Chomp has been grappled for one round, the Armagedadon may attempt to chew the creature as a bonus action. The target must succeed a DC 15 dexterity saving throw or take 26 (4d12) piercing magic damage. On a successful save, the target only takes half damage (2d12).

RESPONSE: Arcane Shield. Enter the description for your action.

The Armagedadon can take 4 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 Armagedadon regains spent legendary actions at the start of its turn.

ORDER: Load Dread Cannon (Costs 1 Action). Loads one ammo type into the dread cannon and primes it for firing, allowing the Armagedadon to make an attack with the Dread Cannon after using this. If an ammo is switched, it takes 1 combat round to replenish. Choose from the following list:

Caustic Round: +4 to hit, reach 50-200 ft., 1 target. On impact, deal 51 (8d10+7) poison damage and the target becomes poisoned. Leaves a poison trail as it flies, any creature that goes within 5 ft of the fumes receive the Poisoned condition. 

Homing Rounds: casts magic missile at the 8th level.

Tsunami Round: shoots a supersonic torpedo that turns all the water in a 60ft radius of the Armagedadon into difficult terrain. When a creature that is not the Armagedadon attempts to enter, it must succeed a DC 18 strength saving throw or get sucked into the fast moving currents. On a failed save, take 21 (6d6) bludgeoning damage.

Flash Round: shoots a grenade at a target. After traveling 100 ft, the grenade can explode in a 40ft radius, dealing 9 (2d8) radiant damage and 9 (2d8) thunder damage. Each creature within the blast radius must make a DC 21 constitution saving throw or become blinded and deafened for 2 rounds.