Myrmyxicus

 Myrmyxicus large fiend (demon), chaotic evil

Armor Class 18 (natural armor)
Hit Points 210 (20d10+100)
Speed 30 ft., swimming 60 ft., flight 10 ft.


STR 20/+5
DEX 18/+4
CON 21/+5
INT 19/+4
WIS 15/+2
CHA 14/+2

Saving Throws STR +11, CON +11
Skills Athletics +11, Intimidation +8, Perception +8 
Damage Resistances Cold, Fire, Lightning; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks
Damage Immunities Poison
Condition Immunities Poisoned

Languages Abyssal, telepathy 120 ft.
Senses Darkvision 120 ft., Passive Perception 18
Proficiency Bonus +6

Magic Resistance. The myrmyxicus has advantage on saving throws against spells and other magical effects.

Magic Weapons. The myrmyxicus' weapon attacks count as magical

Actions

Multiattack. The myrmyxicus uses Grasping Spout and makes three attacks: one with its bite and two with its claws. The demon can replace a bite with spellcasting.

Spellcasting. The demon can cast the following spells without material components. Intelligence is the spellcasting ability (DC 18)

At will: true sight, freedom of movement, telekinesis, fear

1/day each: control weather, dispel magic, finger of death, teleport

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 30 (4d12 + 4) piercing damage.

Unholy Scythe. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 18 (4d6 + 4) slashing damage.

Grasping Spout. The myrmyxicus magically launches a spout of water at one creature it can see within 60 feet of it. The target must make a DC 17 Strength saving throw, and it has disadvantage if it’s underwater. On a failed save, it takes 22 (4d8 + 4) acid damage and is pulled up to 60 feet toward the myrmyxicus. On a successful save, it takes half as much damage and isn’t pulled.

Undertow. As a bonus action when the myrmyxicus is underwater, it can cause all water within 60 feet of it to be difficult terrain for other creatures until the start of its next turn.