Disclaimer: This is not your avarage Monster, and its CR was designed, for my Homebrew Games, witch contain more altered than unaltered rules, classes and mechanics. In "base game" D&D this is perhaps more than just a 16 in CR, maybe lets say 20 (I haven't played without my hombrew in years so this may be horribly wrong, it also is still the 2014 stuff I'm considering).
It also has a lot of mechanics and may not be advised for both beginner DM's or Parties to run this, just because you need to consider a lot of factors. I will try to answer upcoming questions.

Optional Path - Cleansing: The Players can try to get to the noble heart of the angel and make him realize the horror of his existence. If the Abbot does not know of the harming any civilians, they are actively opposing strahd, are doing a good Job protecting Irene and have not taken any of the pacts from the Amber temple entities (all of this "to his knowledge").
Then the characters can try to talk to the Abbot and make him realize his own crimes. If they talk to him they can, bring up to topic (and if they are convincing enough) they can attempt a DC 30 Persuasion Check. On a fail the Abbot is insulted, and will attack, if he favors his odds, or thinks conflict is unavoidable. On a success the Abbot considers their Proposal, inviting them, to discuss the matter further, after which they continue talking to him in the garden. There they have to pass further persuasion, checks with a DC starting at 28. This DC is lowered by 2 for every success and increased by 2 for every fail. If the DC exceeds 30 they fail and it ends in a similar situation as if they had failed the first roll. If succeed on the roll for DC 20 the Abbot is convinced by their arguments. But for every roll the PC's make the abbot he returns, with a question of his own, questioning their morality and their actions in Barovia. Should their actions differ from the good they are trying to convince him of, they may tell the truth, which increases the DC by 2, or they can try to lie, making a deception check, with the same DC as the current Persuasion check. On a fail, the DC for the next persuasion check increases by 4, otherwise, the DC lowers by 2 again. Should they convince him, that that what he is doing is wrong, there are several options. If they convince him of a honor based morality, he will immediately leave to fight Strahd, offering the characters and their allies to come with him. If they convince him by a mercy based morality, he heals the transfigured humans, then tries to kill himself. If they convince him using pride, in the sense of "how dare you let yourself get bossed around by Strahd", he will fight Strahd with them, if he thinks they have a good chance of success, but will turn on them, once he is destroyed. Of course there are many other options, that you may choose to implement once that stage has been reached.

How to Run this Monster: This is a version of the Abbot, that leans more into the body manipulating side of him. He is a necromancer, taking inspiration from Mahito (JJK reference), if that description helps you. The idea is, that you can do a lot of things, using the body transformations, but he is definitely played the most optimally by staying out of the heaviest of the fighting, leaving that for his transfigured humans to do, and controlling the battlefield using his spells, abilities and his transformation, whilst flooding the battlefield with his transfigured humans, of which he should have some already nearby in the Abby and even more as reinforcement. Maybe like a vanguard of 10 and reinforcements like 30? That would increase the difficulty of the fight by a bit, especially depending on the capability of the players to avoid being swarmed by them, adding maybe 1-4 to the CR of the encounter, depending on how many players they are, wether they bring other allies, how capable they are at dealing and evading hordes, and how susceptible they are to swarm tactics. If they just nuke all of them into the ground with cones of could and fireballs, they add like 1 in CR, if they have only the paladin with his smites, or they only have a druid to tank, then this will add like a 4 CR to this encounter, especially considering that the Abbot has AOE healing abilities, meaning that this would be quite the battle of attrition. What you need to watch out for is a teleporting paladin, because 154 HP isn't alot, when confronted with two 3rd level smites. He could go down in like 2 rounds, or less if the paladin has more help and stacked magic items. So keep in mind that the Abbot knows that, and would not stay anywhere, where he knows the paladin can get him. Which doesn't mean that, he can't go into melee. Especially isolated casters, are something that he should go after, not just with his minions but also by himself, in order to reduce the AOE damaging capabilities of the Party or the parties healing. A rule of thumb would be, asking yourself the question "does this target want to be in melee combat" if that answer is "no" that the Abbot should consider going after it, if it doesn't put him into danger, if the answer is "yes!" he should be avoiding it. Note that these considerations have to be taken from his perspective, as you the DM have a lot of knowlege, which he would not have and of course he can make mistakes. Also consider running away, if that would fit how you think the Abbot would act.

Hyper-magical field: (8-th level spell, 1 Action, Range)
A 10 ft. radius sphere surrounds you. This field is suffused with the magical energy that flows through the multiverse. Until the Spell ends, the sphere moves with you, centered on you.
Within this High magical field gain the following effects:

• Target effects: Spells and other magical effects that target a number of creatures, have their number of targets doubled.
• Areas of Magic: The area of Spells cast within the Hyper-magical field are doubled.
• Magic Items: The properties and powers of magic items are doubled within a Hyper-magical field are doubled ((+2) becomes (+4), etc), all expended charges of magic items replenish at the beginning of the attuned persons turn, or at initiative count 20.
• Maximum Output: Instead of rolling for damage, the damage of spells, magical effects, magic reliant effects and magic reliant abilities, are not rolled, instead you take the maximum. Any non damaging effects are doubled.
• Anti-Anti-Magic: If this spells area overlaps with an anti-magical field of equal level, both cancel out. If an anti-magical field of any other level overlaps with this spells area, than the spell with the higher level gets its radius reduced by the difference in their level (9th level hyper magical field - 8th level anti magical field -> 8th level hypermagical field and anti-magical field is dispelled).
• Overcast: If you cast this spell with a spell slot of 9th (or higher if you have something like that) its area is increased by 10ft per spells level above 8.

Adaptive Ward: (4th Level Spell, 1 Action, Range: 9m, )
You raise a magical shield, whenever damage is dealt, to you or to an ally within the shield you may roll 4d6 and reduce the damage taken by that amount.
If you cast this spell with a Spellslot of 5th or higher level, you may reduce the damage taken by additional 1d6 per level above the 4th.

True Ward: (7th Level Spell, 1 reaction, Range: 18m, instantaneous)
When a creature would deal damage you can force the creature to make a Wisdom saving throw, on a fail, that dealt damage becomes 0. On a successful save that damage is halved instead.

Antimagic Field: If Upcast, increase area by 10ft per level upcast.

Body-Transformation:
The following transformations are possible using the Shape-Manipulator feature. This list can be expanded of course. The DC for all Saves that are required by these traits are 10+ the CR of the Monster (15) -> DC(25). The list lists a feature in bold and than (examples) in brackets, where as the example can both be a physical feature and animals that possess such a tool.
Note that  I am NOT a Biologist, merely enjoy watching documentaries and designing monsters.

• Grippers: (Tentacles, Mandibles..., like Octopus, some insect species like ants)
  If a creature is hit with a melee attack, that creature has to succeed on a Str- or Dex-SV, in order to avoid being grappled. A creature grappled by this effect can as a bonus action make an Athletics or Acrobatics check in order to brake free on a success.
• Armor: (Plating, scales, chitin armor..., as in ankylosaurs, lizards, or insects)
  The AC of this creature increases by its PB
• Punishment: (venomous/poisonous coating, extreme tempratures, acids..., as in poison dart frogs)
  If a creature attacks the monster in the protected area, or touches it, it takes damage equal to its PB. (May involve weak spots, if you want)
• Especially large: (like really big..., plesiosaurs and other sauropods, elefants, hippos, whales)
  The target gains additional HP equal to twice its CR.
• Built-in-weapon: (claws, sharp teeth..., lions and many other carnivorous animals)
  The creature has one such built in weapon, which still counts as a "weaponless attack", but deals (half the creatures PB) in d8s as damage.
• Especially dangerous weapons: (like a really really long claw, or a heavy tail..., like an ancylosaurs tail, a thagomizer, or a saber-tooth cats saber-teeth)
  If the creature attacks a creature with an attack roll it can reduce the hit roll by its PB, and deal twice its PB damage on a hit.
• Regeneration: (like trolls in Warhammer, or Vampires)
  At the beginning of their turn, they regain HP equal to its CR. Additionally the creature gains a bonus to death saves equal to its PB.
• Special adaption: (Insert Extremophile species here)
  The creature gains resistance against a damage type. If the creature already had resistance against that damage type it can gain immunity instead. If another creature deals damage of that damage to that creature, that other creature has to make a Con-SV or take damage equal to the attacked creatures CR.
• Good senses: (good eyes, good nose..., dogs, eagles, owls, bats)
  The creature has advantage on perception checks, provided, that that sense could actually be used to perceive the thing in question. It also gains Proficiency in Perception.
• Very Small: (smol smol ant..., mice, ants)
  The creature loses per CR two HP, but its AC increases by half its PB
• Special Armor: (using some magical armoring, being good at dodging, being to fat to die)
  The Base AC of this creature can be calculated by adding 10+ its DEX mod. + any other ability score
• Special Movement: (Fins, wings, good holding hands..., Monkeys, moles, dolphins, eagles)
  The monster gains an additional movement equal to its first form of movement, or 9m.
• Faster Movement: (fast as f**k boiiii..., Emus, Kangaroo, cheetah)
  One movement rate of the creature doubles.
• Explodes: (Would you like to explode?..., Creeper (aw man))
  As an action, the monster can explode at a time of its choosing forcing every creature within a range of metres equal to its CR, to make a DEX-SV, on a fail they take (PB)d8 in Fire damage and an equal amount of thunder damage. They take only half as much on a failed save. This deals the same damage to the monster, which may not kill it.
• Wild Dash: (bursts of speed..., hares, eagles)
  As an bonus action, the Monster elongates, or absolves a wild dash, up to its CR in metres. If it deformed it reverts to its original form and place after the turn is complete, or if it absolved a dash, it switches position to the space it moved to after the movement.
• Multiple Arms: (Hello there..., General Grievous, Sukuna, also a good fix to make thrii-kreen actually good)
  The Creature can hold more things with more hands. The Monster can, if it used the Attack action this turn, as a bonus action make take an additional attack action with every additional pair of arms, but the number of additional attacks generated by this, may not be grater than the creatures PB. If the creature takes this feature again, it can choose to either gain an additional pair of arms, or an to use the attacks of that pair of arms as part of the action without expending a bonus action.
• Multiple Mouths: (fuga..., Sukuna)
  The Creature has addtional Mouths, with these, the creature it may speak (if the creature is capable of speech). If the creature makes a bite attack, it may bite with both mouths, per attack.

 Feel free to add to this list, and share your additions with me.