Cubemunculus

Motionless Stealth. If the cubemunculus does not move and takes no actions, it becomes invisible at the end of its turn. This invisibility ends if the cubemunculus moves or takes an action. 

Variant Abilities(Roll d6). Cubemunculi often undergo mutations as they age. Listed here are some of the most common of these mutations.

1. Piercing Eyeball. The cubemunculus gains truesight, the ability to see all living creatures within 120 ft., and is immune to the blinded condition.
2. Strangling Mustache. The cubemunculus gains the following attack: 

Strangling Mustache. Melee Weapon Attack: +5 to hit, reach 15 ft., 1 target. Hit: 5 (2d12 + 2) piercing damage. On a hit, the target is grappled until it uses its action to escape. 

1. Bewitching Hair. Any creature that can see the cubemunculus must make a DC 15 Charisma saving throw or be charmed by the cubemunculus for 1 minute. This effect ends automatically if an affected creature takes damage. Once a creature succeeds on their saving throw against this effect or is no longer charmed by the cubemunculus, it is immune to this effect for 1 hour.
2. The Dreaded Song of La Tee Dah. When the cubemunculus takes its dreaded singsong action, its range increases to 300 ft. and it deals an additional 2d6 psychic damage. 
3. Underwater Adaptation. The cubemunculus gains a 30 ft. swimming speed, can breath underwater, and is immune to the environmental hazards of depth. Additionally, the cubemunculus can see clearly at any depth underwater as if it were broad daylight. 
4. Abominable. The cubemunculus gains resistance to cold damage and bludgeoning, piercing, and slashing damage from non-magical weapons.

Dreaded Singsong. As an action, the cubemunculus begins a song that can be heard by any creature within 60 ft. of it that is not deafened. All affected creatures immediately take 2d12 thunder damage, and must roll a DC 15 Wisdom saving throw to resist bursting into fancy dance moves, causing disadvantage on all attack rolls. The singsong lasts until the end of the cubemunculus' next turn.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 5 (1d4 + 2) piercing damage. 

Move it. The cubemunculus causes one incoming attack to miss, alternatively, the cubemunculus completely avoids the effects of a failed Dexterity saving throw.