Antimagic Cone.  The golem's central brain creates an area of antimagic, as in the antimagic field spell, in a 50-foot cone.  At the start of each of its turns, the beholder decides which way the cone faces and whether the cone is active.  The area works against the golem's own spells.

Berserk.  Whenever the golem starts its turn with 40 hit points or fewer, roll a d6.  On a 6, the golem goes berserk.  On each of its turns while berserk, the golem attacks the nearest creature it can see.  If no creature is near enough to move and attack, the golem attacks an object, with preference for an object smaller than itself.  Once the golem goes berserk, it continues to do so until it is destroyed or regains all its hit points.

The golem's creator, if within 60 feet of the berserk golem, can try to calm it by speaking firmly and persuasively.  The golem must be able to hear its creator, who must take an action to make a DC 15 Charisma (Persuasion) check.  If the check succeeds, the golem ceases being berserk.  If it takes damage while still at 40 hit points or fewer, the golem might go berserk again.

Immutable Form. The golem is immune to any spell or effect that would alter its form.

Innate Spellcasting.  The golem's innate spellcasting ability is Wisdom (spell save DC 14, +3 on spell attacks).  It can innately cast the following spells, requiring no material components:

At will: detect magic, detect thoughts, levitate

1/day: dominate monster

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

Magic Weapons. The golem's weapon attacks are magical.  

Psychic Absorption.  Whenever the golem is subjected to psychic damage, it takes no damage an instead regains a number of hit points equal to the psychic damage dealt.

Multiattack. The golem makes two slam attacks, and if it can, uses its mind devour.

Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage.

Mind Devour.  The golem moves towards a hostile creature that it can see within range and makes a melee attack.  If the attack is successful, the target must succeed on a DC 15 Dexterity saving throw; on a failed save, the target becomes grappled by the golem and takes 77 (22d6) psychic damage and becomes unconscious for 1 minute.