Amphibious. The Ytelk'enduss Ohm'dhron can breathe air and water.

Prismatic Membrane. While underwater, Ytelk'enduss Ohm'dhron is resistant to spell damage and other magical effects.

Probing Telepathy. If a creature communicates telepathically with Ytelk'enduss Ohm'dhron, he learns the creature's greatest desires if the aboleth can see the creature.

Immutable Form. Ytelk'enduss Ohm'dhron is immune to any spell or effect that would alter his form.

Magic Resistance. Ytelk'enduss Ohm'dhron has advantage on saving throws against spells and other magical effects.

Powerful Luminosity. Ytelk'enduss Ohm'dhron sheds bright light illuminating in a 30-foot radius. The light is so strong that any creature standing at 5 feet and looking at Ytelk'enduss Ohm'dhron must succeed on a DC 19 Wisdom saving throw or be blinded until the start of the creature's next turn. If the creature decides to not look at the Aboleth it can avoid the saving throw at the start of its turn, but it will have disadvantage on attack rolls against it until the start of its next turn.   

If Ytelk'enduss Ohm'dhron takes more than 25 damage from a single attack, he must make a concentration check. If Ytelk'enduss Ohm'dhron fails the save, the light fades and the ability is reactivated at the end of his next turn.

Prismatic Shield. Only once per turn If Ytelk'enduss Ohm'dhron makes a successful saving throw against a spell, or a spell attack misses him, Ytelk'enduss Ohm'dhron can choose another creature (including the spellcaster) that he can see within 120 feet of it. The spell targets the chosen creature instead of Ytelk'enduss Ohm'dhron. If the spell forced a saving throw, the chosen creature makes its own save. If the spell was an attack, the attack roll is rerolled against the chosen creature.

Multiattack. Ytelk'enduss Ohm'dhron makes three tentacle attacks, or two tentacle attacks and his intoxicating touch attack, or his intoxicating touch attack and his enslave action.

Spellcasting. Ytelk'enduss Ohm'dhron is an 23rd-level spellcaster. His spellcasting ability is Intelligence (spell save DC 20, +14 to hit with spell attacks). Ytelk'enduss Ohm'dhron has the following wizard spells prepared:

Cantrips (at will): mind sliver, viscous mockery, chill touch, dancing light, eldritch blast, poison spray, shape water 

1st level (5 slots): chaos bolt, dissonant whispers, absorb elements, bane, command

2nd level (4 slots): mind spike, phantasmal force, darkness, hold person

3rd level (4 slots): blink, counterspell, dispel magic, pulse wave,

4th level (4 slots): phantasmal killer, confusion, control water, dimension door

5th level (4 slots): synaptic static, dominate person, dream, modify memory

6th level (3 slots): mental prison, forbidden, globe of invulnerability

7th level (2 slots): force cage, power word pain, project image

8th level (1 slots): feeblemind, maddening darkness, tsunami 

9th level (1 slots): psychic scream, weird

Tentacle. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage. If the target is a creature, it must succeed on a DC 19 Constitution saving throw or become diseased. The disease has no effect for 1 minute and can be removed by any magic that cures disease. After 1 minute, the diseased creature's skin becomes translucent and slimy, the creature can't regain hit points unless it is underwater, and the disease can be removed only by heal or another disease-curing spell of 6th level or higher. When the creature is outside a body of water, it takes 6 (1d12) acid damage every 10 minutes unless moisture is applied to the skin before 10 minutes have passed.

Tail. Melee Weapon Attack: +9 to hit, reach 10 ft. one target. Hit: 15 (3d6 + 5) bludgeoning damage.

Intoxicating Touch. Melee Spell Attack: +9 to hit, reach 5 ft., one creature. Hit: The target is magically cursed for 1 hour. Until the curse ends, the target has disadvantage on Wisdom saving throws and all ability checks.

Enslave (5/Day). Ytelk'enduss Ohm'dhron targets one creature he can see within 30 feet of it. The target must succeed on a DC 19 Wisdom saving throw or be magically charmed by Ytelk'enduss Ohm'dhron until the he dies or until he is on a different plane of existence from the target. The charmed target is under Ytelk'enduss Ohm'dhronse control and can't take reactions, and Ytelk'enduss Ohm'dhron and the target can communicate telepathically with each other over any distance.

Whenever the charmed target takes damage, the target can repeat the saving throw. On a success, the effect ends. No more than once every 24 hours, the target can also repeat the saving throw when it is at least 1 mile away from Ytelk'enduss Ohm'dhron.

Fluxscale (Recharge 3-6). As a bonus action at the end of his turn, Ytelk'enduss Ohm'dhron pulses with unstable energy. All creatures within 5 feet of Ytelk'enduss Ohm'dhron or within 15 feet if looking in his direction must make a DC 19 Wisdom saving throw. On a failed save, the creature takes 8d4 psychic damage and takes a -1 penalty to attack rolls, ability checks and saving throws. This effect can stack up to a cap of -6 and lasts until the creature completes a long rest. At the end of each of its turns, the creature can make a DC 19 Wisdom saving throw. On a successful saving throw, the creature removes two effects. Ytelk'enduss Ohm'dhron always uses this action if he can do so.

Psychic Attacker. Once per turn when Ytelk'enduss Ohm'dhron casts a spell that deals psychic damage, he can use it's reaction to treat all 1's on a damage die as a 2.

Psychic Rebuke. If Ytelk'enduss Ohm'dhron is subjected to a spell that would deal psychic damage or teleport him against his will, he can use his reaction to force that creature to make a DC 20 Charisma saving throw. Taking 2d10 psychic damage on a failed save, or half as much damage on a successful one.

Ytelk'enduss Ohm'dhron 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. Ytelk'enduss Ohm'dhron regains spent legendary actions at the start of its turn.

Detect. Ytelk'enduss Ohm'dhron makes a Perception check.

Tail Swipe. Ytelk'enduss Ohm'dhron makes one tail attack.

Potent Spellcasting. Ytelk'enduss Ohm'dhron casts a spell of second level or lower.

Psychic Drain (Costs 2 Actions). One creature charmed by Ytelk'enduss Ohm'dhron takes 12 (2d12) psychic damage, and he regains hit points equal to the damage the creature takes.

Tail Swipe. Ytelk'enduss Ohm'dhron makes two tail attacks.

Potent Spellcasting (Costs 2 Actions). Ytelk'enduss Ohm'dhron casts a spell of third level or lower.

Psychic Torrent (Costs 2 Actions). Ytelk'enduss Ohm'dhron releases a torrent of psychic energy. Each creature in a 120 ft cone must succeed on a DC 20 Intelligence saving throw or take 22 (4d10) 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.

Enslave (Costs 3 Actions). Ytelk'enduss Ohm'dhron uses his Enslave action.