Innate Spellcasting. Fraz-Urb'luu's spellcasting ability is Charisma (spell save DC 23). Demogorgon can innately cast the following spells, requiring no material components:
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. Magic Weapons. Fraz-Urb'luu's weapon attacks are magical. 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.
Multiattack. Fraz-Urb'luu makes three attacks: one with his bite and two with his fists. Bite. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 19 (3d6 + 9) piercing damage. Fist. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 22 (3d8 + 9) bludgeoning damage.
Fraz-Urb'luu 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. Fraz-Urb'luu regains spent legendary actions at the start of his turn. Tail. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) bludgeoning damage. If the target is a Large or smaller creature, it is also grappled (escape DC 24). The grappled creature is also restrained. Fraz-Urb'luu can grapple only one creature with his tail at a time. Phantasmal Killer (Costs 2 Actions). Fraz-Urb'luu casts phantasmal killer, no concentration required.
On Initiative count 20 (losing initiative ties), Fraz-Urb'luu can take a lair action to cause one of the following effects: he can't use the same effect two rounds in a row:
Fraz-Urb'luu's mere presence in a plane other than his own bends it to his shape. His lair is the fane of this change, often it is where he was summoned or where he stays for extended time, his energies pooling around him.
The region containing Fraz-Urb'luu's lair is warped by his magic, creating one or more of the following effects: