Unspeakable Horror (3 Heads)

Spellcasting. The mage is a 9th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 14, +8 to hit with spell attacks). 

Cantrips (at will): fire bolt, Word of Radiance, Druidcraft

2nd level (3 slots): Darkness, Moonbeam

3rd level (4 slots): counterspell, Cure Wounds, Guiding Bolt

Three Minds: You have advantage on Constitution saving throws that you make to maintain your concentration on a spell.

Slasher: Reduce Speed, Once per turn when you hit a creature with an attack that deals slashing damage, you can reduce the speed of the target by 10 ft., until the start of your next turn.

Multiattack. The horror makes two Limbs attacks or Three thrown weapon attacks 

Limbs. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 21 (3d10 + 5) bludgeoning damage.

Bone Blade. The horror’s limb ends in a blade made of bone, which deals slashing damage instead of bludgeoning damage. In addition, it scores a critical hit on a roll of 19 or 20 and rolls the damage dice of a crit three times, instead of twice.

Bone Scimitar. Ranged Weapon Attack: 20/60 +8 to hit, reach 5 ft., one target. Hit: 21 (2d6+5) slashing damage.

Reality-Stealing Hex (Recharge 5–6). The horror expels a wave of perception-distorting energy. Each creature within 30 feet of the horror must make a DC 15 Wisdom saving throw. On a failed save, the target takes 22 (5d8) psychic damage and is deafened until the end of its next turn. If the saving throw fails by 5 or more, the target is also blinded until the end of its next turn.