Here is my CR15 version. My party did not like this.
Banshee
Medium Undead (banshee) –Neutral Evil
Armor Class 17
Hit Points 224
Speed: 0 ft., fly 40ft (hover)
STR DEX CON INT WIS CHA
1 (-5) 14 (+2) 10 (+0) 12 (+1) 20(+5) 18 (+4)
Saving Throws Wis +10, Cha +9
Damage Resistances acid, fire, lightning, thunder; bludgeoning, piercing, and slashing from non-magical weapons
Damage Immunities cold, necrotic, poison
Condition Immunities charmed, exhausted, frightened, grappled, paralysed, petrified, poisoned, prone, restrained
Senses darkvision 60ft., passive Perception 15
Languages Common, Elvish
Challenge 15 (13,000)
Detect Life. The banshee can magically sense the presence of living creatures up to 5 miles away. She knows the general direction they’re in but not their exact location.
Incorporeal Movement. The banshee can move through other creatures and objects as if they were difficult terrain. She takes 5 (1d10) force damage if she ends her turn inside an object.
ACTIONS
Corrupting Touch. Melee Spell Attack: +10 to hit, reach 5ft, one target. Hit: 82 (15d10) necrotic damage.
Horrifying Visage. Each non-undead creature within 60 feet of the banshee that can see her must succeed on a DC 18 Wisdom saving throw or be frightened for 1 minute. A frightened target can repeat the saving throw at the end of each of its turns, with disadvantage if the banshee is within line of sight, ending the effect on a success. If a target’s saving throw is successful or the effect ends for it, the target is immune to the banshee’s horrifying visage for the next 24 hours.
Wail (1/Day). The banshee releases a mournful wail, provided that she isn’t in sunlight. The wail has no effect on constructs and undead. All other creatures within 30 feet of her that can hear her must make a DC 16 constitution saving throw. On a failure, a creature drops to 0 hit points. On a success, a creature takes 55 (10d10) necrotic damage.