Void Mauler

Call of the Void. If the mauler dies, its body is destroyed in a blast of dark energy. Any creature within 10 feet of it when this happens must make a DC 20 STR Save or take 20 (4d10) force damage and is knocked back 10 feet, landing prone.

Siege monster. The mauler deals double damage to objects and structures. 

Otherworldly. The mauler is immune to effects that would change its movement speed, including magical difficult terrain, and cannot be affected by changes in gravity. 

Alien Eye. The mauler has a third eye that is larger than its other two on its forehead, allowing it to automatically succeed on checks to perceive illusions or other distortion effects. The eye is a weak point for the mauler, and attacking it specifically is made with disadvantage, but deals double damage. This weakness in only exploitable via piercing damage. 

Innate Spellcasting. The mauler's spellcasting ability is Intelligence (spell save DC 18, +10 to hit with spell attacks). The mauler can innately cast the following spells, requiring no material components. 

At will: zephyr strike, chaos bolt, command

3/day: eyebite, magic missile (5th level)

1/day: negative energy flood

Magic Weapons. The mauler's attacks are considered magical.

Magic Resistance. The mauler has advantage on saves against spells and magic based effects. 

Legendary Resistance (3/day). If the mauler fails a saving throw, it can choose to succeed instead.

Multiattack. The mauler makes 2 rend attacks. 

Rend. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: (2d12 + 6) slashing damage and 5 (1d10) force damage, and the target must make a DC 20 STR Save or is knocked prone. 

Corrupt (Recharge 6). The mauler attempts to control or destroy the minds of nearby creatures. Up to 5 creatures within 30 ft. of the mauler of its choice must make a DC 20 CHA save or take 24 (4d12) psychic damage and is charmed by it for 1 hour. While a creature is charmed in this way, it is under the complete control of the mauler, and must use its turns to carry out the mauler's will, including attacking the creature's allies. On a success, a creature takes half damage and is not charmed. An affected creature can repeat the save on the end of each of its turns, ending the effect on a success. 

The Void Mauler has 3 legendary actions it can choose from the options below. Each action can only be used after another creatures turn, and all actions are regained at the start of each of the maulers turns. 

Rend. The mauler makes a rend attack.

Spell Attack. The mauler casts a spell. 

Abduct (costs 2 actions). The mauler makes a rend attack. If the attack hits, the creature is grappled (escape DC 19) instead of making a save against being knocked prone.

Destructive Charge (costs 3 actions). The mauler roars, then charges forward 50 feet. Any creature that the mauler crosses with must make a DC 20 DEX Save or it takes (4d12+) bludgeoning damage and 10 (2d10) force damage, and it is knocked prone. Any object or structure the mauler moves through is immediately destroyed.