Unusual Nature. The Beast doesn’t require air, food, drink, or sleep.

Spell Immunity. The Beast is immune to three spells chosen by its creator.

banishment, heat metal, true polymorph.

Siege Monster. The Beast deals double damage to objects and structures.

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

Towering Presence. The Beast can choose any number of creatures within 20 feet of itself to make make a DC 20 Wisdom saving throw, unless The Beast is incapacitated. On a failed save, the creature is frightened, and its speed is reduced to 0 until the start of its next turn. If a creature's saving throw is successful, the creature is immune to The Beast's Towering Presence for the next 24 hours.

Arcane-Empowered Strikes. The Beast's melee attacks are magical

Relentless Will. If damage reduces The Beast to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken. On a success, The Beast drops to 1 hit point instead.

Innate Spellcasting. The Beast's spellcasting ability is Intelligence(spell save DC 21). The Beast can innately cast the following spells, requiring no material components:

At will: locate creature, mind spike(duration is 24 hours)

3/day each: hold monster, freedom of movement, scrying

Multiattack. The Beast makes four attacks with its claws.

Claw. Melee Attack: +14 to hit, reach 10 ft., one target. Hit: 15 (2d8+7) slashing damage.

The Beast can take 1 legendary action, choosing from the options below. Only one legendary action option can be used at the end of another creature’s turn. The Beast regains its spent legendary action at the start of its turn.

Infallible Will. The Beast can choose to end one condition affecting it.

Implacable Pursuit. The Beast moves its speed in a straight line, ignoring difficult terrain. Any creature must succeed on a DC 17 Dexterity saving throw or be knocked prone and takes 32 (5d10 + 7) bludgeoning damage. Any object in its path that isn't being worn or carried also takes this damage. 

Demonic Fury. When The Beast's hit points have been reduced to one quarter it can enter a relentless rage that only ends when it is unconscious. While in this rage The Beast does not differentiate between friend and foe and will attack anything it can sense.