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

Command Minion. As long as there is a controlled minion of Rogthoth's in a 120 ft. Radius, he can control one of them and either use an Action or Movement of the chosen creature.

Legendary Resistance (3/Day). If Rogthoth fails a saving throw, he can choose to succeed instead.

Horrifying Image. At the start of his turn, Rogthoth can look at one creature within 60 ft. and implant an horrible nightmare in their mind. The creature must succeed on a DC 18 Wisdom Saving Throw or sees a terrible illusion in their mind, giving them disadvantage on all their attack rolls for the next turn. If they are a caster, they have to make a Concentration Check DC 12 to be able to cast a spell. If the creature succeeds on the saving throw, they have advantage on it for 24 hours.

Innate Spellcasting (Psionics). Rogthoth’s innate spellcasting ability is Intelligence (spell save DC 19). It can innately cast the following spells, requiring no components:

At will: detect thoughts, levitate, charm person

1/day each: Geas, Mass Suggestion, dominate monster, plane shift (self only)

Rogthoth can attack with his tentacles and use either the Shadow Burst or Mind Blast if available.

Tentacles. Melee Weapon Attack: +7 to hit, reach 5 ft., one creature. Hit: 15 (2d10 + 4) psychic damage. If the target is Medium or smaller, it is grappled (escape DC 15) and must succeed on a DC 15 Intelligence saving throw or be stunned until this grapple ends.

Extract Brain. Melee Weapon Attack: +7 to hit, reach 5 ft., one incapacitated humanoid grappled by the mind flayer. Hit: The target takes 55 (10d10) piercing damage. If this damage reduces the target to 0 hit points, the mind flayer kills the target by extracting and devouring its brain.

Shadow Burst. (Recharge 6).  Rogthoth uses his control over the shadows and let them attack every creature he desires in a 60-foot radius around him. Each targeted creature in that area must succeed on a DC 16 Dexterity saving throw or take 32 (5d10 + 5) piercing damage or half as much on a success.

Mind Blast (Recharge 5–6). The mind flayer magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 16 Intelligence saving throw or take 22 (4d8 + 4) psychic damage and be stunned for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

As a Bonus Action, Rogthoth can swap places with one of his mind controlled slaves. This does not provoke an attack of opportunity if he gets out of melee range from a creature that engaged him in combat.

Rogthoth can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Rogthoth regains spent legendary actions at the start of his turn.

Place Swap. Rogthoth can swap places with one of his mind controlled slaves ina 120 ft. radius.

Raise Undead. Rogthoth can raise up to 1d4 Undead, as long as these creatures were not raised before.

Brain Parasite (2 Actions). Rogthoth uses his brain parasites to attack one creature he can see within 60 feet. This creature has to succeed on a DC 15 Wisdom saving throw or is infested with a brain parasite. After 1d4 days the parasite takes full control of his host, if it isn't removed as long as Rogthoth is alive.