Arcanaphage

Arcane Discharge. When an arcanaphage dies, it explodes in a surge of partially-digested magical energy. Each creature within 5 feet per Feed score must make a DC 14 Dexterity saving throw, taking 3 (1d6) force damage per Feed score on a failed save, or half as much damage on a successful one. For 1 minute afterward, the affected area is awash with volatile magic. A creature that starts its turn in the affected area takes 7 (2d6) force damage.

Hunger. Each time it feeds in combat, it regains hp equal to twice the level of the spell it ate and increases its Feed score by 1. The arcanaphage can't have a Feed score higher than 8, and its Feed score reduces by 1 each time it finishes a long rest.

Ingest Magic. At the start of each of the arcanaphage's turns, each creature within 30 feet of it that is currently maintaining concentration on a spell must make a DC 14 Constitution saving throw. On a failure, the creature's spell ends and the arcanaphage feeds.

Magic Immunity. The arcanaphage is immune to damage from spells. It has advantage on saving throws against all other magical effects.

Multiattack. The arcanaphage makes two tentacle attacks.

Tentacle. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage.

Voracious. The arcanaphage's tentacles glow when a spell is cast within 30 feet of it, countering the spell. This reaction works like the counterspell spell, except the arcanaphage must always make a spellcasting ability check, no matter the spell level. Its ability check for this is +5. If it successfully counters the spell, the arcanaphage feeds.