Arcane Devourer

Null Aura. The Devourer's latent energies fill the world around them draining those within range of their willpower. Each creature that starts its turn within 20 feet of the devourer must make a DC 16 Wisdom saving throw. Unless the save succeeds, the creature rolls a d8 to determine what it does during the current turn:

1–4: The creature is stunned until the end of the turn.

5–6: The creature is frightened until the end of the turn and uses its movement to get as far as possible from the nullifier.

7-8: The creature loses its lowest level spell slot. This spell slot can be used to fuel arcane consumption.

Arcane Consumption. The Devourer can convert stolen magical energy taken through through its Null Aura to cast any spell the target creature could cast with that spell slot. For example if a cleric with the spell heal lost a spell slot that would allow them to use heal then the Spirit Devourer could use that spell slot to cast Heal.

Innate Spellcasting. The devourer’s innate spellcasting ability is Charisma (spell save DC 16). It can innately cast the following spells, requiring no material components:

At will: counterspell (see “Reactions” below), detect magic, dispel magic, see invisibility

1/day: antimagic field

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

Multiattack. The devourer makes two attacks with its claws.

Claws. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 11 (2d6 + 4) slashing damage plus 11 (2d10) force damage, and the target is grappled (escape DC 16) if it’s a creature. The nullifier has two claws, each of which can grapple one creature.

Shapechanger. The Devourer can use an action to change their appearance and their voice. They determine the specifics of the changes, including coloration, hair length, gender identify, etc. They may adjust their height and weight and can change size between medium and small. They can mimic the appearance of another race though no game statistics change. They can duplicate the appearance of another individual they have seen. They must adopt the same basic arrangement of limbs as they have. 

Counterspell. The Devourer attempts to interrupt a creature that it can see within 60 feet in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, the nullifier makes a Charisma check with a DC equal to 10 + the spell’s level. On a success, the creature’s spell fails and has no effect.