What is the proper CR for this homebrewed Souls-esque NPC?

Question

Returning for another year of a gothic horror adventure, I wanted to try my hand at creating a truly difficult encounter. Following on past my original npcs I opted to turn a certain Dark Souls boss into a DnD npc. However, my bravado got the better of me. And now I am forced to accept that I may have designed this npc in a way that makes them way too small a challenge for a group of mid-level players. I was aiming for the CR 12-15 range.

What is the proper CR for this creature given their abilities?

Once again, the calculator just doesn't seem to cut it. There's a lot of interactions between different abilities and legendary actions. It's enough to make my head spin and I just can't wrap my head around it all. But for completeness sake, the calculator says CR 12 based off a CR 10 Defense and CR 13 Offense.

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Sister Lorelei

Medium undead, neutral

Armor Class: 17 (natural armor)
Hit Points: 90 (20d8)
Speed: 40 ft.

STR - 11 (+0)
DEX - 19 (+4)
CON - 10 (+0)
INT - 14 (+2)
WIS - 15 (+2)
CHA - 20 (+5)

Saving Throws: Dex +9, Con +5, Cha +10
Skills: History +5, Insight +7, Religion +5, Stealth +9
Damage Resistances
Damage Immunities: Necrotic, Poison; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks
Condition Immunities: Charmed, Exhaustion, Frightened, Paralyzed, Poisoned
Senses: Passive Perception 12
Languages: The languages they knew in life
Challenge: ???

Traits

Magic Resistance. Sister Lorelei has advantage on saving throws against spells and other magical effects.

Nimble Escape. Sister Lorelei can take the Dash, Disengage or Hide action as a bonus action on each of their turns.

Rejuvenation. If Sister Lorelei is destroyed, she gains a new body in 24 hours if their frozen heart is intact, regaining all her hit points and becoming active again. The new body appears within 5 ft. of sister lorelei's frozen heart.

Spellcasting. Sister Lorelei is a 14th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks).
Sister Lorelei has the following sorcerer spells prepared:
Cantrips (at will): chill touch, control flames, frostbite, ray of frost, shape water, true strike
1st level (4 slots): burning hands, detect magic, jump
2nd level (3 slots): mirror image, misty step
3rd level (3 slots): fireball, haste
4th level (3 slots): greater invisibility, wall of fire
5th level (2 slots): cone of cold, enervation
6th level (1 slot): investiture of ice
7th level (1 slot): fire storm

Turn Resistance. Sister Lorelai has advantage on saving throws against any effect that turns undead.

Actions

Multiattack. Sister Lorelei makes two attacks with her spectral sickle and one attack with her reaping scythe.

Reaping Scythe. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 9 (1d10+4) slashing damage + 7 (2d6) necrotic damage

Spectral Sickle. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 6 (1d4+4) piercing damage + 7 (2d6) cold damage.

Legendary Actions

Sister Lorelei can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. Sister Lorelei regains spent legendary actions at the start of its turn.

Cantrip Sister Lorelei casts a cantrip.

Move Sister Lorelei moves up to their speed without provoking opportunity attacks

Reaping Scythe Sister Lorelei makes a Reaping Scythe attack.

Spectral Evanescence Sister Lorelei becomes magically invisible for up to 1 minute or until immediately after they make an attack roll.

Answer 1

Ability Breakdown

To start, we have AC 17, 90 hp, +9 to attack/DC 18 saving throws, and roughly 42 damage per round. Let's go through abilities and see how those get modified, using the guidelines on DMG 274-281.

First, her three saving throws increase her effective AC by 2 (19).

Next, we have her damage immunities. Your expected CR of 12-15 means that her immunities should increase her effective HP by half (135).

Magic Resistance increases effective AC by 2 (21).

Nimble Escape increases effective AC and attack bonus by 4 (25 and +13), assuming the monster hides every round. I'll revisit this when I look at Spectral Evanescence.

Rejuvenation doesn't affect CR- it affects adventure design more than combat.

We'll come back to Spellcasting momentarily.

Turn Resistance doesn't affect CR.

DPR: Spellcasting and Legendary Actions

The next step is calculating actual DPR. This is where Spellcasting and legendary actions come in. Over the first three rounds of combat, let's assume Sister Lorelei opens with Fire Storm, then two Cones of Cold. AoEs should be assumed to hit two characters, but Fire Storm is very permissive with its AoE so let's bump it to 3. In total that's 21d10 fire damage plus 32d8 cold damage, for an average of 260 damage over three rounds, or 87 DPR.

Now let's consider legendary actions. Reaping Scythe is the most damaging of these, dealing 16 damage. Three legendary actions increases DPR by 48, to 135.

Summary

All put together, that's AC 25, 135 hp, +13 to hit/DC 18 saves, and 135 DPR. She has a Defensive CR of 10 (5 for hp, +5 for an AC 10 higher than expected) and an Offensive CR of 21 (20 for DPR, +1 for an attack roll higher than expected), for an average of CR 15.

Final Thoughts

So, on paper, she's a CR 15 monster. But the math isn't the whole story: two main concerns present themselves.

One: she's far, far more powerful offensively than defensively. With the damage output of a CR 20 monster, if she goes first she'll melt through your party horrifyingly quickly unless they're extremely lucky, skilled, tanky, or some combination of the three. A CR 12-15 monster should have somewhere in the range of 75-100 DPR, so 135 is a lot. This isn't really helped by the fact that she has the hp of a CR 5 monster (CR 1 if you disregard her immunities, as a party with magical weapons will). If the party gets the upper hand on her with a surprise round or a crit or two, they may wipe her out before she gets to pull her big guns. Her overpowering offense and atrocious hp, in concert, pose a threat of making the fight very swingy, depending on who gets to go first. I'd strongly recommend keeping your Offensive and Defensive CRs much closer together.

Two: Spectral Evanescence. Lorelei can become invisible three times per round for free, and more importantly doesn't break invisibility when she uses her most damaging attacks, those big AoE spells. She's essentially going to be invisible all the time- so every attack on her has disadvantage, if they can even track her.

That said, all attacks on her having disadvantage was accounted for in Nimble Escape, and she'll need to break invisibility to use her damaging Legendary Actions. If she doesn't (or if she uses Frostbite, which doesn't require an attack roll) she'll reduce her DPR by a massive amount. The party will just have to Ready actions to attack when she stabs someone and becomes visible for a turn.

All in all, Spectral Evanescence is strong (and probably quite frustrating for the party) but I don't think it's completely broken. To be safe, I'd modify it to also break invisibility whenever she casts a spell, like the normal Invisibility spell. That way she'd need to spend Legendary Actions every turn to disappear again, cutting into her DPR.

Answer 2

By the DMG: CR 15

She has 20 offensive and 10 defensive CR for an average of 15.

Damage output: Offensive CR 20 48 damage per turn by legendary actions. For straight damage, reaping scythe is better than cantrips, although their additional properties might make them worthwhile from time to time. Maximum damage output with first three actions:

• Fire Storm deals 38 damage against two targets by the book. That’s 76 damage.
• Cone of Cold: 36 (5th) or 40 (6th) damage to two targets, i.e. 72 or 80.
• Wall of Fire: (5th level) 20 damage to two targets, i.e. 40, plus some potential additional damage on subsequent turns.
• Upcasted fireball (5th level): 35 damage to two targets, i.e. 70.

Maximum damage for the first three rounds is Fire Storm + Cone of Cold (6th) + Fireball (5th). This is a total of 76 + 80 + 70 damage, i.e. 226, or 75 on average. With the legendary actions, this is 123 damage per turn. This would be CR 20 by DMG p. 274.

The save DC is 18. The attack bonus is +10. But, between Nimble Escape, Greater Invisibility and Spectral Evanescence, she has lots of options to gain disadvantage, which counts as +4 to attack (DMG p. 281). This would make it +14 total and increase offensive CR to 22. Since the best options available rely on saving throws, however, the DC 18 might be more relevant, which is okay for CR 20.

Hit Points and armor class: Defensive CR 10

90 hit points is CR 2. For the expected challenge rating of 12-15, we need to add 50 % hp for the immunities (DMG p. 277), so 135. That would be CR 5. She should have 15 AC.

She has AC 17. Since she can easily gain disadvantage on opponent’s attacks (same methods as advantage above), this becomes 21. Magic Resistance increases it to 23. Another +2 for saving throw proficiencies for 25. This increases the defensive CR by 5.

The final defensive CR is 10.

Is this the truth?

• The damage output is terrific: Not only can she do the indicated damage in the first three rounds of combat; burning through her fourth level slots she can almost maintain it for another three rounds using upcasted fireballs.
• That said, she is a glass cannon (vide supra). It might happen that she doesn’t survive this long.
• The targeting method of fire storm makes it almost impossible not to hit the entire party, further increasing the damage output.
• Obviously the DMG monster building rules are not that accurate, it is entirely possible that a monster is easier than the table says.
• However, you monster is very skewed: To high offense, and in the defense, to low hit points.
• That means someone is going to die. And fast. Who that is will be mostly be decided by the dice: initiative and attack rolls / saving throws on the first turns.
• Her hit points are very low. She has lots of defensive potential to avoid damage but when they fail some time, she’s going to fall like wheat before a sickle blade (pun intended). Also, the nonmagic weapon immunity seems very much negligible. Unless you have someone, who cannot reasonably deal magic damage this will have very little impact: At most the damage she doesn’t take while players figure out the immunity.
• You should have a look at the sidebar on DMG p. 82 on high CR monsters (compared to PCs). Your total CR seems fine, but too high an offensive rating has the same effect, in the end. Moreover, you have the same problem the other way around: Your party’s offensive CR is also way too high for poor Lorelei. One might be tempted to assume that things will even out, but they don’t, it just makes the affair incredibly swingy.
• I’ll finish with some pieces of personal advice: I have tried out monsters with low hp and high AC (or AC equivalent) multiple times and it never works out if you exaggerate. Either the calculation is off because it is not actual AC (I’m pretty sure this is the case here) or it becomes an unfun drag where attacks do not hit.
• Second, your statblock is way complicated. If I would use it, I would sure forget stuff or mix things up. Maybe cut some of the things which are not so relevant.
• Probably it is fixable, if you just cut the spellcasting above level 4 and double her hit points or similar. Just a rough suggestion, I didn’t do the math.