Incorporeal Movement. The demonic nun can move through other creatures and objects when it is on the Material Plane, and vice versa.

Magic Resistance. The demonic nun has advantage on saving throws against spells and other magical effects.

Spellcasting. The demonic nun is a 16th-level spellcaster. Its spellcasting ability is wisdom (spell save DC 17, +8 to hit with spell attacks). The demonic nun has following warlock spells prepared:

Cantrips (at will): chill touch, infestation, toll the dead

1st level (4 slots): hellish rebuke, witch bolt, hex, arms of hadar

2nd level (3 slots): darkness, mind spike, mirror image

3rd level (3 slots): hunger of hadar, counterspell, fear

4th level (3 slots): shadow of moil, blight, banishment

5th level (2 slots): negative energy flood, synaptic static

6th level (1 slot): circle of death

7th level (1 slot): power word pain

8th level (1 slot): maddening darkness

1/day: plane shift

Ethereal Sight. The demonic nun can see 60 feet into the Ethereal Plane when it is on the Material Plane, and vice versa.

Multiattack. The demonic nun makes two unarmed strike attacks.

Unarmed Strike. Melee Weapon Attack: +9 to hit, reach 5 ft., one creature. Hit: 8 (1d8 + 4) bludgeoning damage. 

Call Shadows. The demonic nun targets one Medium or smaller creature that casts a shadow. The target's shadow must be visible to the demonic nun and within 30 feet of her. If the target fails a DC 17 Charisma saving throw, its shadow detaches from it and becomes a shadow that obey's the demonic nun's commands, acting on initiative count 20. A greater restoration spell or a remove curse spell cast on the target restores its natural shadow, but only if its undead shadow has been destroyed. 

Possession (Recharge 6). One humanoid that the demonic nun can see within 5 feet of it must succeed on a DC 17 Charisma saving throw or be possessed by the demonic nun; the demonic nun then disappears, and the target is incapacitated and loses control of its body. The demonic nun now controls the body but doesn't deprive the target of awareness. The demonic nun cannot be targeted by any attack, spell, or other effect, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunity to being charmed and frightened. It otherwise uses the possessed target's statistics, but doesn't gain access to the target's knowledge, class features, or proficiencies.

The possession lasts until the body drops to 0 hit points, the demonic nun ends it as a bonus action, or the demonic nun is turned or forced out by an effect like the dispel evil and good spell. When the possession ends, the demonic nun reappears in an unoccupied space within 5 feet of the body. The target is immune to the demonic nun's Possession for 24 hours after succeeding on the saving throw or after the possession ends.

Etherealness. As an action, the demonic nun enters the Ethereal Plane from the Material Plane, or vice versa. It is visible on the Material Plane while it is in the Border Ethereal, and vice versa, yet it can't affect or be affected by anything on the other plane.