Chupoclops

Aura of Despair. Each creature within 30 feet of Chupoclops makes Wisdom saving throws and death saving throws with disadvantage, and regains half the normal number of hit points from any healing.

Well of Souls (Mythic Trait; Recharges after a Short or Long Rest). If Chupoclops is reduced to 0 hit points, it doesn’t die or fall unconscious. Instead, it regains 490 hit points and is surrounded by a swarm of partially-devoured souls. Whenever a creature within 30 feet of Chupoclops hits it with an attack, the swarm retaliates, dealing 11 (2d10) necrotic damage to the attacker. Additionally, Chupoclops can now use the options in the “Mythic Actions” section. Award a party an additional 75,000 XP (150,000 XP total) for defeating Chupoclops after it uses Well of Souls.

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

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

Magic Weapons. Chupoclops' weapon attacks are magical.

Soul Sense. Chupoclops can sense the location of any creature within 120 feet of it, unless that creature's type is construct.

Spider Climb. Chupoclops can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Web Walker. Chupoclops ignores movement restrictions caused by webbing.

Multiattack. Chupoclops can use its Frightful Presence. It then makes three attacks: one with its bite, two with its forelegs. It can use its Swallow instead of its bite. 

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 35 (4d12 + 9) piercing damage, and the target must make a DC 23 Constitution saving throw, taking 18 (4d8) poison damage on a failed save, or half as much damage on a successful one. If the target is a creature, it is grappled (escape DC 24). Until this grapple ends, the target is restrained, and Chupoclops can't bite another target.

Foreleg. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 18 (2d8 + 9) slashing damage.

Frightful Presence. Each creature of Chupoclops' choice within 120 feet of it and aware of it must succeed on a DC 23 Wisdom saving throw or become frightened for 1 minute. A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. These later saves have disadvantage if Chupoclops is within line of sight of the creature.

If a creature succeeds on any of these saves or the effect ends on it, the creature is immune to Chupoclops' Frightful Presence for the next 24 hours.

Innate Spellcasting. Chupoclops casts one of the following spells, requiring no components and using Charisma as the spellcasting ability (spell save DC 23):

At will: blink, dream, etherealness, weird

Soul Web (Recharge 5-6). Chupoclops weaves a web of ethereal silk that fills a 30-foot cube next to it. The web is difficult terrain, its area is lightly obscured, and it lasts for 1 minute. Any creature that moves into the web or that starts its turn there must make a DC 23 Constitution saving throw. On a failed save, the creature takes 18 (4d8) necrotic damage. This webbing is immune to all damage except radiant damage. A 5-foot cube of the web is destroyed if it takes at least 20 radiant damage on a single turn.

Chupoclops 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. Chupoclops regains spent legendary actions at the start of his turn.

Foreleg. Chupoclops makes a foreleg attack.

Ethereal Jaunt. Chupoclops casts etherealness.

Move. Chupoclops moves up to its speed without provoking opportunity attacks.

Bite (Costs 2 Actions). Chupoclops makes one bite attack or uses its Swallow.

If Chupoclops' Well of Souls trait is active, it can use the options below as legendary actions.

Devour Soul. Chupoclops devours a soul from its ghostly shroud. The next melee attack that Chupoclops makes before the end of its next turn deals an extra 44 (8d10) psychic damage on a hit.

Soul Web (Costs 2 Actions). Chupoclops recharges Soul Web and uses it.

Enervating Web (Costs 3 Actions). Each creature in a space filled with Chupoclops's Soul Web must succeed on a DC 23 Constitution saving throw or suffer one level of exhaustion.