Arcane Corruption Beast

Limited Magical Immunity: The beast can't be affected or detected by Spells of 3rd level or lower unless it wishes to be. It has advantage on Saving Throws against all other Spells and magical Effects.

Arcane Sense. Can sense spell casters and magical creatures within 60 feet.

Tendrils (Recharge 5-6). The creature's tendrils can be attacked directly. Each tendril has AC of 11 and HP of 12. Any damage done to the tendril is also dealt to the main beast. If a tendril is killed than it retracts back into the body. A piece of arcane ooze is left on whatever the tendril was touching. The creature can have no more than 3 tendrils at a time

Ooze.  Any creature with arcane ooze on its body has disadvantage on dexterity saving throws against a tendril grapple.

The Beast has three tendrils that spring from its form that it can be used to perform any of the following tendril attacks. Each tendril can do its own action.

Tendril Grapple. A tendril lashes out up to 30 feet and attempts to grapple a creature. The target must make a dexterity saving throw contested by the monster's athletics check. If the target fails, they are restrained. The target can attempt to break the grapple by making a contested strength check or by killing the tendril.

Tendril Pull. A tendril currently restraining a creature can attempt to pull the creature up to 15 feet toward the monster. The monster and target must make contested strength checks.

Tendril Drain. A tendril currently restraining a creature can attempt to drain the creature. The target must succeed on a DC 15 wisdom saving throw. A creature suffers 15 (3d8+2) psychic damage and is unable to cast a spell until the end of their next turn on a failed save or half on a successful one and their spell-casting is not affected.

Tendril Whip. Melee Attack: +8 to hit, reach 30 ft., one target. Hit:  11 (1d12 + 5) slashing damage.

Bite. Melee Attack: +8 to hit, range 5 ft., one target. Hit: 18 (2d12 + 5) piercing damage. 

Engulf. The arcane corruption beast attempts to engulf a creature within 5 ft. The target must succeed on a DC 15 wisdom saving throw or be completely absorbed into the body of the beast. An engulfed creature is blinded, deafened, restrained, and can't breathe. The beast drains the magic of an engulfed creature. If the targets falls to 0 HP, it becomes an arcane zombie.