Antarcanum

Unusual Nature. The Antarcanum does not require sleep, food, or water. 

Phasic Body. As an action, the Antarcanum can choose to enter the Ethereal Plane until the start of its next turn. While in the Ethereal Plane, the Antarcanum can pass through walls and is immune to all forms of damage. Additionally, regardless of the plane of existence the Antarcanum resides upon, it can pass through Walls of Force and Prismatic Walls without making any saving throws or additional checks. The Antarcanum suffers no effects of either of these spells.

Magic Consumption. When the Antarcanum is targeted by a spell or magical effect, the spell has no effect. Instead, the Antarcanum heals either an amount equal to 5 * the level of the spell cast or 25 hitpoints if the magical effect is not a leveled spell. 

Magical Resistance. The Antarcanum has advantage on saving throws against all spells or magical effects, Additionally, if the Antarcanum would normally take half damage upon succeeding on a saving throw against a spell, they instead take no damage. 

Limited Telepathy. The Antarcanum can magically transmit simple messages and images to any creature within 120 feet of it that can understand a language. This form of telepathy doesn't allow the receiving creature to telepathically respond.

Multiattack. The Antarcanum makes three tentacle attacks and a single grasping claw attack. It then uses Maddening Presence. 

Grasping Claw. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 15 (3d6 + 4) bludgeoning damage. If the target is a creature, it must succeed on a DC 19 Strength saving throw against the Antarcanum. On a failed save, the creature becomes grappled and restrained. The creature can repeat the saving throw at the end of their turn. 

Tentacle. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 13 (2d8 + 4) piercing damage. Additionally, if the creature is able to cast spells, the creature must make a saving throw with a DC equal to 8 plus the amount of damage taken by the tentacle attack. On a failed save, the creature loses one spell slot of the highest level they have remaining. The creature then takes force damage equal to 5 * the level of the spell slot consumed. If the creature has no spell slots remaining, they take no additional damage. 

Maddening Presence. The Antarcanum reveals its true nature to all that witness it. Every creature that can see the Antarcanum within 150 ft. must make a DC 13 Intelligence saving throw. On a failure, the creature takes 20 points of psychic damage and must make a DC 13 Intelligence check. On a successful Check, the creature becomes immune to this effect for 24 hours. On a successful Intelligence Saving throw, the creature takes damage equal to half of the Antarcanum's remaining hitpoints and becomes stunned for 1 minute. The creature can attempt to break free of the stun at the beginning of their turn by repeating the saving throw. 

The Antarcanum can take two reactions a round, but may only use one reaction a turn. 

Arcane Rending. When a creature with 90 ft. of the Antarcanum casts a spell, the Antarcanum can use their reaction to counter the spell. If the spell is 5th level or lower, the spell automatically fails. If the spell is 6th level or higher, the Antarcanum must make an Arcana check with a DC equal to 10 plus the spells level. If the spell is countered, the Antarcanum heals an amount equal to the spells level.