Legendary Resistance (3/Day).

If Demogorgon fails a saving throw, he can choose to succeed instead.

Magic Resistance.

Demogorgon has advantage on saving throws against spells and other magical effects.

Two Heads.

Demogorgon has advantage on saving throws against being blinded, deafened, stunned, or knocked unconscious.

Actions

Multiattack.

Demogorgon makes two Tentacle attacks. He can replace one attack with a use of Gaze.

Tentacle.

Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 28 (3d12 + 9) force damage. If the target is a creature, it must succeed on a DC 23 Constitution saving throw, or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if its hit point maximum is reduced to 0.

Gaze.

Demogorgon turns his magical gaze toward one creature he can see within 120 feet of him. The target must succeed on a DC 23 Wisdom saving throw or suffer one of the following effects (choose one or roll a d6):

1–2: Beguiling Gaze. The target is stunned until the start of Demogorgon's next turn or until Demogorgon is no longer within line of sight.
3–4: Confusing Gaze. The target suffers the effect of the confusion spell without making a saving throw. The effect lasts until the start of Demogorgon's next turn. Demogorgon doesn't need to concentrate on the spell.
5–6: Hypnotic Gaze. The target is charmed by Demogorgon until the start of Demogorgon's next turn. Demogorgon chooses how the charmed target uses its action, reaction, and movement.

Spellcasting.

Demogorgon casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 23):

At will: detect magic, major image
3/day each: dispel magic, fear, telekinesis
1/day each: feeblemind, project image

Legendary Actions

Demogorgon can take 2 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. Demogorgon regains spent legendary actions at the start of its turn.

Gaze. Demogorgon uses Gaze and must use either Beguiling Gaze or Confusing Gaze.
Tail. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) bludgeoning damage plus 11 (2d10) necrotic damage.
Cast a Spell (Costs 2 Actions). Demogorgon uses Spellcasting.

Lair Actions

On initiative count 20 (losing initiative ties), Demogorgon can take one of the following lair actions; he can't take the same lair action two rounds in a row:

Darkness. Demogorgon casts the darkness spell four times, targeting different areas with the spell. Demogorgon doesn't need to concentrate on the spells, which end on initiative count 20 of the next round.
Illusory Duplicate. Demogorgon creates an illusory duplicate of himself, which appears in his space and lasts until initiative count 20 of the next round. On his turn, Demogorgon can move the illusory duplicate a distance equal to his walking speed (no action required). The first time a creature or an object interacts physically with Demogorgon (for example, by hitting him with an attack), there is a 50 percent chance that the illusory duplicate is affected, not Demogorgon, in which case the illusion disappears.

Regional Effects

The region containing Demogorgon's lair is warped by his magic, creating one or more of the following effects:

Beguiling Realm. Within 6 miles of the lair, all Charisma (Persuasion) and Charisma (Performance) checks have disadvantage, and all Charisma (Deception) and Charisma (Intimidation) checks have advantage.
Frenzied Animals. Beasts within 1 mile of the lair become frenzied and violent—even creatures that are normally docile. Within that area, any ability check involving Animal Handling has disadvantage.
Venomous Beasts. The area within 6 miles of the lair becomes overpopulated with poisonous snakes and other venomous Beasts.

If Demogorgon dies, these effects fade over the course of 1d10 days.