Corrupted Fiend

Devil's Sight. The Corrupted Fiend's sight isn't impeded by magical darkness.

Magic Resistance. The Corrupted Fiend has advantage on saving throws against spells and other magical effects.

Magically Enhanced. The Corrupted Fiend's attacks are considered magical.

Innate Spellcasting. The Corrupted Fiend spellcasting ability is Charisma (spell save DC 18, +11 to hit with spell attacks). The Corrupted Fiend has following spells prepared:

At will: detect thoughts, disguise self, mage hand, minor illusion

3/day each: charm person, detect magic, invisibility, major image, suggestion

1/day each: dominate person, fly, plane shift, true seeing 

Multiattack: The Corrupted Fiend makes three attacks, one with its Bite and two with its Claws. Alternatively, it can replace one of its Claw attacks with a Flamberge attack, or cast a Spell.

Bite: Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 9 (2d4+5) piercing damage, and the target must make a DC 15 CON Saving throw, or take another 9 (2d6+2) Poison damage.

Claw: Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 11 (2d6+5) slashing damage and 15 (2d10+5) Necrotic damage. Also on hit: The target is afflicted with a curse that negates any resting bonuses, both from Short and Long rests. This curse can be removed through the casting of a 5th level Remove Curse spell.

Flamberge: Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 13 (1d8+8) Slashing and 3 (1d6) Fire damage. Target must also make a DC 16 DEX saving throw or be caught on fire. This fire lasts for 1d4+1 turns, and deals 1d6+3 points of fire damage at the start of every turn thereafter. Additionally, a creature may spend and action to put out a creature if it is on fire.

Spell: The Corrupted Fiend casts one of its spells.