Astral Avatar (Mythic Trait; Recharges after a Short or Long Rest). If Ilsensine is reduced to 0 hit points, it doesn’t die or fall unconscious. Instead, it regains 400 hit points. In addition, Ilsensine fuses the astral plane with the landscape around it. For 1 hour, during which time it can use the options in the "Mythic Actions" section, the area in a 100-foot radius around Ilsensine became a distorted. The area becomes part of the Astral plane. All terrain, objects, and creatures remain the same, but all creatures are subject to the conditions of the Astral plane as described in chapter 2 of the Dungeon Master's Guide. In Ilsensine leaves the area, the effect is ended early.

If a creatures leaves the radius, they must make a DC 27 charisma saving throw. On a successful save, the creature decides whether they end up in the Astral plane or the Material plane, either way appearing in the location they would have if they normally walked out of the area in the single plane. On a failed save, the location is determined randomly. 

At the end of the duration, all creatures in the area make a DC 27 charisma saving throw. On a failed save, the creature is teleported to Ilsensine's choice of the material plane of the Astral plane. On a successful save, the creature decides. 

Tendrils of Thought. Extending from Ilsensine are ten tendrils of pure thought, which it can use as both weapons and tools. Each tendril represents a different creature that existed at some point. They can think and experience just like as if they were actually alive. 

Each tendril is has a base length of 20 feet. Ilsensine can move around the tendrils as if they were normal limbs. When a tendril comes within 20 feet of a creature (other than Ilsensine) for the first time or ends its turn there, the creature must make a DC intelligence saving throw. On a failed save, Ilsensine can hear all it's thoughts as if it was thinking them, until the start of Ilsensine's next turn. On a successful save, the effect is revered the creature can hear Ilsensine's thoughts until the start of Ilsensine's next turn. 

The tendrils can be attacked as if they were creatures. They have an armor class of 22, 50 hit points, and immunity to psychic damage. When they are reduced to 0 hit points, that are destroyed. 

Creature Sense. Ilsensine is aware of the presence of creatures within 100 miles of it that have an Intelligence score of 4 or higher. It knows the distance and direction to each creature, as well as each one’s Intelligence score. It can also sense the surface emotions (such as anger or joy) of each creature in the radius. A creature protected by a mind blank spell, a nondetection spell, or similar magic can’t be perceived in this manner.

Psychic Defense. While Ilsensine is wearing no armor and wielding no shield, its AC includes its Intelligence modifier.

Legendary Resistance (5/Day). If Ilsensine fails a saving throw, it can choose to succeed instead.

Limited Magic Immunity. Unless it wishes to be affected, Ilsensine is immune to spells of 6th level or lower. It has advantage on saving throws against all other spells and magical effects.

Telepathic Hub. Ilsensine can use its telepathy to initiate and maintain telepathic conversations with up to ten creatures at a time. Ilsensine can let those creatures telepathically hear each other while connected in this way.

Psionic Concentration. When Ilsensine casts a spell that requires concentration, it can instead choose a creature under the effects of its Telepathic Hub feature. That creature concentrates on the spell instead of Ilsensine, as if that creature had cast the spell. That creature chooses whether or not to end concentration, must make concentration saves when it takes damage, and cannot concentrate on any other spells. Likewise, Ilsensine does not need to make concentration saves, nor is it repented from casting other spells that require concatenation. 

Innate Spellcasting (Psionics). Ilsensine is a 20th-level spellcaster. Its innate spellcasting ability is Intelligence (spell save DC 15; +7 to hit with spell attacks). It can innately cast the following spells, requiring no components:

At will: guidance, mage hand, vicious mockery, true strike

1st level (at will): charm person, command, sanctuary, shield

2nd level (at will): mirror image, suggestion, levitate

3rd level (at will): clairvoyance, fear, major image, counterspell

4th level (3 slots): confusion, stone shape, greater invisibility

5th level (3 slots): scrying, telekinesis, modify memory

6th level (3 slots): contingency, harm

7th level (3 slots): plane shift, project image, teleport

8th level (2 slots): dominate monster, feeblemind

9th level (2 slots): astral projection, weird

Mutliattack. The Avatar of Ilsensine can use its frightful presence. Then, it makes three attacks with its tendrils of thought. If it has less than three tendrils remaining, it instead makes a number of tendril attacks equal to its the number of tendrils it has remaining.

Tendril of Thought. Melee Weapon Attack: +19 to hit, reach equal to the length of the tendril ft., one target. Hit: 33 (6d10) psychic damage. If this damage reduces the target to 0 hit points, the target is instantly killed and transformed into a tendril of thought. Ilsensine regains hit points equal to the amount of damage it dealt, and gains another tendril of length 10 feet which represents the creature Ilsensine just killed. Additionally, Ilsensine learns any information that creature knew. 

Frightful Presence. Ilsensine exudes its aberrant presence. Each creature of its choice that is within 240 feet of it and aware of it must succeed on a DC 27 Wisdom saving throw or become frightened for 24 hours. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to Ilsensine's Frightful Presence for the next 24 hours.

Create Minion. Ilsensine forms ones of its tentacles into the fragment of a creature. It's tentacle becomes a creature with the same appearance as the creature the tentacle represented, which appears in an unoccupied space within 10 feet of Ilsensine. Ilsensine can control it at will (requiring no action), and can see and hear from the illusion's eyes and ears. 

The creature has an armor class of 20, 10 hit points, and immunity to psychic and poison damage. If the creature is reduced to 0 hit points, it disappears. When this happens, Ilsensine can force all creatures who caw it at any point to succeed on a DC 27 wisdom save or completely forget the existence of the creature. 

At any time (no action required) Ilsensine can turn the creature back into a tentacle. 

Psychic Explosion (Recharge 5-6). Ilsensine detonates one of its tentacles in an all-consuming wave of thought. Ilsensine looses one of its tendrils, and all creatures in a 80 foot sphere centered on the tip of the tendril must make a DC 27 intelligence saving throw, taking 95 (19d8+10) psychic damage on failed save or half as much on a successful one. A creature who fails the save is also stunned for a minute. A creature stunned in this way can repeat the save at the end of each of its turns, ending the effect early on a success.

Extend Tendril. Ilsensine magically manipulated the length of its testicles. It chooses one of its tendrils, which grows or shrinks by 10 feet (its choice), to a maximum length of 1000 feet. When Ilsensine completes a long rest, the length of each of its tendrils resets to 20 feet.

Aberrant Form. Ilsensine extrudes its form of dread. Each creature frightened of it must succeed on a DC 27 wisdom save or take 18 (4d8) psychic damage. A creature who can see Ilsensine has a disadvantage on the saving throw. 

Ilsensine can take 5 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. Ilsensine regains spent legendary actions at the start of its turn.

Tentacle. Ilsensine makes one tendril attack.

Minor Psionics. Ilsensine casts a spell of 3rd level or lower using its psionics trait. 

Fold Reality (Costs 3 Actions). Ilsensine folds the fabric of reality around itself. Ilsensine teleports to an unoccupied space it can see within 30 feet of itself. 
Until the start of its next turn, it can use its reaction at any time to teleport back to its original location. If it is targeted by an attack when it uses this reaction, the attack misses. 

If Ilsensine's Astral Avatar trait has activated in the last hour, it can use the options below as legendary actions.

Psionics (Costs 2 actions). Ilsensine casts a spell using its psionics trait. 

Psychic Wind (Costs 3 Actions). Ilsensine conjures a psychic wind. All creatures in 100 foot-foot sphere centered on Ilsensine must make a DC 27 intelligence saving throw. On a failed save, a creature must roll on the mental effects table for psychic winds, described in chapter 2 of the Dungeon Master's Guide. 

Mind Screen (Costs 4 Actions). Ilsensine targets a creature it can see within 120 feet of itself. That creature must succeed on a DC 27 wisdom saving throw or completely forget of the existence of the Ilsensine until the start of its next turn. during this duration, the creature cannot see, hear, or feel Ilsensine, nor detect it in any way. The target treats Ilsensine as if it didn't exist, and creates rational explanations for any illogical outcomes that come as a result of not detecting its existence. In addition, the target looses all memory of Ilsensine while the spell lasts. They cannot remember Ilsensine, anything it did or said, or associate any events with it. 

This effect ends early if Ilsensine damages the target.