Immortal Rage (Recharges after a Short or Long Rest). If the Echo would be reduced to 0 hit points, its current hit point total instead resets to 500 hit points, it recharges its Otherworldly Revenge, and it regains any expended uses of Legendary Resistance. Additionally, the Echo can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 120,000 XP (240,000 XP total) for defeating the Echo after its Immortal Rage activates.

Spellcasting. The Echo is a 18-level spellcaster. Its spellcasting ability is Charisma (spell save DC 26, +19 to hit with spell attacks). The Echo has following wizard spells prepared:
Cantrips (at will): chill touch, frostbite, infestation, ray of frost, toll the dead 

1st level (4 slots): burning hands, chromatic orb, ice knife, ray of sickness

2nd level (3 slots): dragon's breath, shadow blade, shatter

3rd level (3 slots): fireball, lightning bolt, vampiric touch

4th level (3 slots): blight, Evard's black tentacles, sickening radiance

5th level (3 slots):, cone of cold, immolation, enervation

6th level (1 slot): investiture of ice

7th level (1 slot): symbol

8th level (1 slot): maddening darkness

9th level (1 slot): blade of disaster

Regeneration. The Echo regains 30 hit points at the start of its turn.

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

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

Multiattack. The Echo can use its Frightful Presence. It then may make three more attacks: two with its icy aura and one with its claws.

Claw. Melee Weapon Attack: +19 to hit, reach 5 ft., one target. Hit: 21 (3d6 + 10) slashing damage plus 14 (4d6) necrotic damage.

Icy Aura. Ranged Weapon Attack: +19 to hit, range 50/200 ft., one target. Hit: 17 (2d6 + 10) cold damage.

Frightful Presence. Each creature of the Echo's choice that is within 120 feet of the Echo and aware of it must succeed on a DC 21 Wisdom saving throw or become frightened 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. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Echo's Frightful Presence for the next 24 hours.

Otherworldly Revenge (Recharge 5–6). The Echo calls upon the darkest reaches of the multiverse, and bends them to its will. The Echo releases energy in a 90-foot line. Each creature in that area must make a DC 26 constitution saving throw, taking 95 (31d6) necrotic damage on a failed save, or half as much damage on a successful one.

The Echo can take 5 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The Echo regains spent legendary actions at the start of its turn.

Draining Shadows. The Echo casts finger of death.

Wisp of Darkness (Costs 2 Actions). The Echo Takes 1d6 damage, and recharges its Otherworldly rage.