Neck

• Amphibious. The Neck can breathe air and water.
• Innate Spellcasting. The Neck's innate spellcasting ability is Charisma (spell save DC 14 Charisma). He can innately cast the following spells, requiring no material components:
  
  At will: dancing lights, minor illusion, calm emotions
• Mimicry. The Neck can mimic animal sounds and humanoid voices. A creature that hears the sounds can tell they are imitations with a successful DC 14 Wisdom (Insight) check.
• Commanding Performance. The Neck can perform a perfect song, mimicking the sound of any instrument. This acts as the command spell (spell save DC 14 Charisma) and affects creatures within a 30 ft radius who can hear.
• Drowning Grapple. The Neck gains advantage when grappling creatures under the affects of his Commanding Performance.

Actions

• Multiattack. The worm makes two claw attacks.
• Claw. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) slashing damage.
• Invisible Passage. The Neck magically turns invisible until he attacks or casts a spell, or until his concentration ends (as if concentrating on a spell). While invisible, he leaves no physical evidence of his passage, so he can be tracked only by magic. Any equipment he wears or carries is invisible with him.
• Illusory Appearance. The Neck covers himself and anything he is wearing or carrying with a magical illusion that makes him look like another creature of his general size and humanoid shape. The illusion ends if the Neck takes a bonus action to end it or if he dies. The changes wrought by this effect fail to hold up to physical inspection. For example, the Neck could appear to have smooth skin, but someone touching him would feel his rough flesh. Otherwise, a creature must take an action to visually inspect the illusion and succeed on a DC 20 Intelligence (Investigation) check to discern that the Neck is disguised.