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.
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 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
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.
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.
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.