Legendary Resistance (3/Day).

If Fraz-Urb'luu fails a saving throw, he can choose to succeed instead.

Magic Resistance.

Fraz-Urb'luu has advantage on saving throws against spells and other magical effects.

Undetectable.

Fraz-Urb'luu can't be targeted by divination magic, perceived through magical scrying sensors, or detected by abilities that sense demons or Fiends.

Actions

Multiattack.

Fraz-Urb'luu makes one Bite attack and two Fist attacks, and he uses Phantasmal Terror.

Bite.

Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 19 (3d6 + 9) force damage.

Fist.

Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 22 (3d8 + 9) force damage.

Phantasmal Terror.

Fraz-Urb'luu targets one creature he can see within 120 feet of him. The target must succeed on a DC 23 Wisdom saving throw, or it takes 16 (3d10) psychic damage and is frightened of Fraz-Urb'luu until the end of its next turn.

Spellcasting.

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

At will: alter self (can become Medium when changing his appearance), detect magic, dispel magic, phantasmal force
3/day each: mislead, programmed illusion, seeming
1/day each: modify memory, project image

Legendary Actions

Fraz-Urb'luu can take 3 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. Fraz-Urb'luu regains spent legendary actions at the start of its turn.

Tail. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) force damage. If the target is a Large or smaller creature, it is also grappled (escape DC 24), and it is restrained until the grapple ends. Fraz-Urb'luu can grapple only one creature with his tail at a time.
Terror (Costs 2 Actions). Fraz-Urb'luu uses Phantasmal Terror.

Lair Actions

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

Conjure Walls and Doors. Fraz-Urb'luu causes up to five doors within the lair to become walls and an equal number of doors to appear on walls where there previously were none.
Psychic Anguish. Fraz-Urb'luu creates a wave of anguish. Each creature he can see within the lair must succeed on a DC 23 Wisdom saving throw or take 33 (6d10) psychic damage.
Simulacrum. Fraz-Urb'luu chooses one Humanoid within the lair and instantly creates a simulacrum of that creature (as if created with the simulacrum spell). This simulacrum obeys Fraz-Urb'luu's commands and is destroyed on the next initiative count 20.

Regional Effects

The region containing Fraz-Urb'luu'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 Wisdom (Insight) checks have disadvantage, and all Charisma (Deception) and Charisma (Performance) checks have advantage.
Nostalgic Pangs. Sapient creatures within 1 mile of the lair frequently see hallucinations of long-dead friends and comrades that vanish after only a brief glimpse.
Twisted Paths. Roads and paths within 6 miles of the lair twist and turn back on themselves, making navigation in the area exceedingly difficult.

If Fraz-Urb'luu dies, these effects fade over the course of 1d10 days.