Art of the Old Ones. Whenever the eldritch lich casts a spell, including Eldritch Blast, it can immediately teleport up to 30 feet to an unoccupied space it can see and it can cause slimy tendrils to manifest from the spell's magic, which strike one creature it can see within 60 feet of it. The creature must succeed on a DC 27 Charisma saving throw or take 14 (4d6) psychic damage and be frightened until the end of its next turn.

Feeding on the Weak. If the eldritch lich drops to 0 hit points while a creature is restrained by a parasitic tentacle, the restrained creature must make a DC 27 Constitution saving throw. On a failure, it drops to 0 hit points and the eldritch lich regains hit points equal to the amount of hit points the creature had.

Forbidden Knowledge. The eldritch lich can maintain concentration on two spells at once.

Eldritch Mind. The eldritch lich is immune to divination magic. When a creature deals psychic damage to the eldritch lich, that creature takes the damage instead, the eldritch lich takes none of the damage. In addition, when a creature tries to read the eldritch lich's thoughts, it must succeed on a DC 27 Wisdom saving throw or gain a short-term madness.

Gateway to Another World. The eldritch lich is considered both an aberration and an undead. When the eldritch lich is reduced to 0 hit points and its Feeding on the Weak fails, more and more tentacles burst forth from the remains of its body  over the next minute, while it dissolves into a pool of inky black liquid serving as a portal to the Far Realms. From the portal an enormous otherworldly horror, to which the tentacles belong, emerges.

When this happens, any creature within 120 feet of the eldritch lich must succeed on a DC 27 Wisdom saving throw or be frightened for 1 minute and gain a long-term madness. A creature frightened in this way remains frightened until the long-term madness ends.

Three successful dispel magic spells cast on the portal, treating it as a 10th level spell, can close it before the eldritch horror emerges. Otherwise, the eldritch horror spawns as a kraken or similar monster per DM's discretion, with the Eldritch Lich's damage resistances and immunites, condition immunities and mental stats.

Innate Spellcasting. The eldritch lich's spellcasting is Charisma (spell save DC 27). It can innately cast the following spells, requiring no components:

• at will: arcane eye, detect thoughts, eldritch blast (4 beams), invisibility (self only), scrying, synaptic static
• 2/day: mental prison, finger of death, feeblemind, psychic scream
• 1/day: dominate monster, power word kill,  wish

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

Master of the Unliving. Any undead ally within 60 feet of the eldritch lich cannot be frightened, is immune to any effect that turns undead and has advantage on all saving throws; in addition that ally regains 20 hit points whenever it starts its turn there.

Spellcasting. The eldritch lich is a 20th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 27, +17 to hit with spell attacks). It knows the following spells:

Cantrips (at will): chill touch, mage hand, mind sliver, prestidigitation

1st to 5th level (8 5th-level slots): animate dead, counterspell, danse macabre, dream, dominate person, Evard's black tentacles, hunger of hadar, negative energy flood, Raulothim's psychic lance,  shield

Turn Immunity. The eldritch lich is immune to any effect that turns undead.

Mutliattack. The eldritch lich attacks with its eldritch blast (4 beams) and makes one attack with each parasitic tentacle, starting out with one attack and increasing whenever it creates a new tentacle either by killing a creature with a tentacle or by using its Spawn of the Far Realms legendary action. It then uses its Paralyzing Glare, Psychic Whispers or casts a spell.

Eldritch Blast (Cantrip). Ranged Spell Attack: +19 to hit, range 300 feet., one creature. Hit: 15 (1d10+10) force damage and the target is pushed up to 10 feet away from the eldritch lich or pulled up to 10 feet towards the eldritch lich.

Paralyzing Glare. Ranged Spell Attack: +19 to hit, range 60 ft., one creature. Hit: 18 (4d8) necrotic damage plus 18 (4d8) cold damage. The target must succeed on a DC 27 Constitution saving throw or be paralyzed until the end of its next turn.

Parasitic Tentacle. Melee Weapon Attack, +19 to hit, reach 30 feet, one target. Hit: 27 (6d8) necrotic damage and the eldritch lich regains hit points equal to the necrotic damage dealt. The target is grappled (escape DC 22) by the eldritch lich. Until this grapple ends, the target is restrained and cannot regain hit points. The grappled target moves with the eldritch lich, and the eldritch lich cannot use its tentacle on another creature until the grapple ends.

A creature that starts its turn grappled by the parasitic tentacle takes 27 (6d8) necrotic damage and the eldritch lich regains hit points equal to the necrotic damage dealt. When a creature dies while restrained by the tentacle, its soul is devoured by the parasite and its body dissolves into a gibbering mouther under the eldritch lich's control, which has a parasitic tentacle growing from within its fleshy mass similar in appearance to the eldritch lich's tentacle. This happens even if the creature was a summon that would otherwise simply dissappear upon death. The eldritch lich can control this tentacle as if it was its own parasitic tentacle. Each such tentacle can grapple one creature. The tentacles act as extensions of the eldritch lich, thus cannot be seperately targeted nor affected by status conditions.

Psychic Whispers. The lich targets one creature it can see within 120 feet of itself. The target must succeed on a DC 27 Wisdom saving throw or take 45 (10d8) psychic damage. as incomprehensible whispers fill the target's mind. Moreover, on its next turn, it must choose whether it gets a move, an action, or a bonus action; it gets only one of the three.

Reality-Warping Teleport (3/day).  The eldritch lich teleports, along with any equipment it is wearing or carrying, up to 60 feet to an unoccupied space it can see. Each creature within 15 feet of the space the eldritch lich left must make a DC 27 Strength saving throw. On a failed save, a creature takes 27 (6d8) force damage and is pulled straight toward the space the eldritch lich left and knocked prone, ending in an unoccupied space as close to its former space as possible. On a successful save, the creature takes half as much damage and isn't pulled.
A creature that fails the saving throw by 5 or more falls through tears in the fabric of reality caused by the eldritch lich's teleport and is banished to the far realms. It reappears at the end of its next turn the space the eldritch lich left or in an unoccupied space as close to it as possible, taking 27 (6d8) psychic damage and gaining a long-term madness from whatever it saw and experienced in the far realms.

Consume Knowledge. Each creature restrained by a parasitic tentacle must make  a DC 27 Intelligence saving throw. On a failure, its Intelligence score is reduced by 1d6. The target falls unconscious if this reduces its Intelligence to 0, otherwise the reduction lasts until cured by [spell]greater restoration/spell] or similar magic. In addition, on a failed save, the eldritch lich learns a spell of its choice known or prepared by the target. It can cast the spell once within the next 24 hours without expending a spell slot. The target loses knowledge of the spell and becomes unable to cast it until it finishes a long rest.

The eldritch lich can take a total of three reactions per round, although only one per turn.

Eldritch Counterspell (1/round). The eldritch lich casts counterspell using one of its spell slots. If the eldritch lich successfully causes a spell to fail, that spell's caster takes psychic damage equal to five times the spell's level and must succeed on a DC 27 saving throw using its spellcasting ability or be unable to cast spells until the start of its next turn.

Instill Madness. In response to being hit by an attack, the eldritch lich utters incomprehensible words towards the attacker. The attacker must make a DC 27 Wisdom saving throw or take 14 (4d6) psychic damage and must use its reaction to move as far as its speed allows away from the eldritch lich.

The eldritch lich 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. The eldritch lich regains spent legendary actions at the start of his turn.

Attack. The eldritch lich uses its eldritch blast or makes one attack with its paralyzing glare or with a parasitic tentacle.

Cast a Spell. The eldritch lich casts a spell.

Command Undead. The eldritch lich utters a dreadful command. Up to six undead friendly to it that can hear it and are within 60 feet of the eldritch lich must use their reactions to move up to their speed and to make an attack against a creature of the eldritch lich's choice.

Summon Undead (Costs 2 Actions). The eldritch lich summons up to six wraiths or greater zombies in unoccupied spaces within 30 feet of it. These undead are twisted and mutated by otherworldly magic, have immunity to psychic damage, can, as part of their multiattack action, cast mind sliver and eldritch blast as if they were 5th-level casters at will using Constitution as their spellcasting ability and can speak telepathically to creatures within 30 feet of them. They remain until destroyed. Undead summoned in this way roll initiative, act on the next available turn, and obey the eldritch lich. The eldritch lich can have no more than six undead summoned by this ability at a time.

Spawn of the Far Realms (Costs 3 Actions, Recharge 5–6). The eldritch lich targets either an undead friendly to it or any other creature. The target must succeed on a DC 27 Constitution saving throw or a seed of the eldritch lich's parasite is implanted within that creature's body and mind. An undead friendly to the eldritch lich automatically fails the saving throw. At the start of the target's turn, it takes 27(6d8) necrotic damage and 27 (6d8) psychic damage as a parasitic tentacle bursts forth from its chest, which is under the eldritch lich's control and acts independently of the host creature - including being able to attack and grapple its own host. The tentacle can be removed by a wish spell or a greater restoration spell cast at 9th level. It also withers away when the eldritch lich dies.
If a creature hosting a tentacle dies, its soul is devoured by the parasite and its body dissolves into a gibbering mouther under the eldritch lich's control, with the parasitic tentacle continuing to exist.