Cerberus Abomination

Legendary Resistance (3/Day). If the Cerberus Abomination fails a saving throw, they can choose to succeed instead.

Burden of Time. Beasts and humanoids have disadvantage on saving throws while within 10 feet of this abomination.

Soul Thirst. When it reduces a creature to 0 hit points, the abomination can gain temporary hit points equal to half the creature’s hit point maximum. While the abomination has temporary hit points from this trait, it has advantage on attack rolls.

Weight of Ages. Any Beast or Humanoid (except an elf) that starts its turn within 5 feet of the abomination has its speed reduced by 20 feet until the start of that creature’s next turn.

Innate Spellcasting. The abomination's innate spellcasting ability is Charisma (spell save DC 16, +8 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: arcane eye, speak with dead

1/day each: arcane gate, bane, compulsion, confusion, true seeing

Spellcasting. The face of Anka on this flesh abomination is a 12th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 16, +8 to hit with spell attacks). It regains its expended spell slots when it finishes a short or long rest. It knows the following warlock spells:

Cantrips (at will): chill touch (3d8 damage), eldritch blast (3 beams, +4 bonus to each damage roll), minor illusion, prestidigitation

1st−5th level (3 5th-level slots): armor of Agathys, blight, darkness, dream, invisibility, fear, hypnotic pattern, major image, contact other plane, vampiric touch, witch bolt

 Berserk. Whenever the abomination starts its turn with 100 hit points or fewer, roll a d6. On a 6, the abomination goes berserk. On each of its turns while berserk, the abomination attacks the nearest creature it can see. If no creature is near enough to move to and attack, the abomination attacks an object, with a preference for an object smaller than itself. Once the abomination goes berserk, it continues to do so until it is destroyed or regains all its hit points.

Immutable Form. The abomination is immune to any spell or effect that would alter its form.

Absorption. Whenever the abomination is subjected to necrotic or psychic damage, it takes no damage and instead regains a number of hit points equal to the necrotic and psychic damage dealt.

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

Magic Weapons. The abomination’s weapon attacks are magical.

 All Together:  This abomination can ONLY do three attacks max per turn.  When faces are gone, the abomination can only slam.

Sad Face: Casts one spell that takes 1 action to cast or uses Shadow Spear.

Shadow Spear (Sad Face). Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) piercing damage, or 8 (1d8 + 4) piercing damage if used with two hands, plus 26 (4d12) necrotic damage.

Happy Face. The abomination attacks with Spiked Chain.

Spiked Chain (Happy Face). Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 10 (2d6 + 3) piercing damage, and the target must succeed on a DC 14 Dexterity saving throw or suffer one additional effect of the abomination's choice:

• The target is grappled (escape DC 14) if it is a Medium or smaller creature. Until the grapple ends, the target is restrained, and the abomination can’t grapple another target.
• The target is knocked prone.
• The target takes 22 (4d10) necrotic damage.

Angry Face. The abomination strikes with a shadow dagger.

Shadow Dagger. Melee or Ranged Weapon Attack: +7 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 13 (4d4 + 3) piercing damage plus 19 (3d12) necrotic damage, and the target has disadvantage on saving throws until the end of the abomination's next turn. Hit or Miss: The dagger magically returns to the abomination’s hand immediately after a ranged attack.

Spellcasting. The abomination casts one of the following spells, requiring no material components and using Intelligence as the spellcasting ability (spell save DC 16):

1/day each: bestow curse, finger of death, gaseous form, seeming

Wave of Weariness (Recharge 4–6). The abomination emits weariness in a 60-foot cube. Each creature in that area must make a DC 16 Constitution saving throw. On a failed save, a creature takes 45 (10d8) psychic damage and suffers 1 level of exhaustion. On a successful save, it takes half as much damage and doesn’t gain a level of exhaustion.

Slam (Either Face). Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 16 (2d10 + 5) bludgeoning damage.

Activate Face: The abomination activates faces between Anka and Maku. 
Shadow Jump (Happy Face): As a bonus action, the abomination can teleport up to 30 feet to an unoccupied space it can see. Both the space it teleports from and the space it teleports to must be in dim light or darkness. The abomination can use this ability between the weapon attacks of another action it takes.

Misty Escape (Sad Face Active. Recharges after a Short or Long Rest). When the abomination takes damage while the face of Anka is activated, it turns invisible and teleports up to 60 feet to an unoccupied space it can see. It remains invisible until the start of its next turn or until it attacks or casts a spell.

3 Legendary Actions

Activate Face: The abomination activates faces between Sad, Happy, and Angry. 1 Action

Cast Cantrip (Sad Face): Can cast a cantrip.  1 Action

Shadow Jump (Happy Face): The abomination can teleport up to 30 feet to an unoccupied space it can see. Both the space it teleports from and the space it teleports to must be in dim light or darkness. The abomination can use this ability between the weapon attacks of another action it takes.  1 Action

Attack (Depends on face): Can attack using either Spiked Chains or Shadow Spear   or Shadow Dagger 1 Action

 On Death Mask Release: The abomination explodes releasing necrotic energy. Anyone within 30ft of it becomes possessed by one of the three masks. (The character is possessed by a masked entity until freed by dispel evil and good or similar magic or they're incapacitated. Whenever the possessed character rolls a 1 on an attack roll, ability check, or saving throw, the makes takes control of the character and determines the character’s behavior. At the end of each of the possessed character’s turns, he or she can make a DC 18 Charisma saving throw. On a success, the character regains control until he or she rolls another 1.)