Void Incursion. The very presence of Xe'noth draws the energies of the Void onto whichever plane it currently inhabits; this alters the environment around Xe'noth to mimic the Void more and more as time passes.

Legendary Resistance (3/Day). If Xe'noth fails a saving throw, it may choose to succeed instead.

Spellcasting. Xe'noth is a 20th level spellcaster. Its spellcasting ability is Charisma (spell save DC 27, +19 to hit with spell attacks). Xe'noth has the following spells prepared:
1st level (At Will): Arms of Hadar, Cause Fear
3rd level (8 slots): Fear
4th level (7 slots): Banishment
5th level (6 slots): Enervation, Negative Energy Flood
6th level (5 slots): Eyebite, Mental Prison, Scatter
7th level (4 slots):Forcecage, Plane Shift
8th level (3 slots):Feeblemind, Maddening Darkness
9th level (2 slots): Psychic Scream

Multiattack. Xe'noth can use its Frightful Presence or Maddening Whisper. It then makes three attacks: Two with its arms and one with its maw.

Slam. Melee Weapon Attack: +19 to hit, reach 15 ft., 1 target. Hit: 36 (4d12 + 10) Bludgeoning damage. Any creature that is struck by this attack must succeed on a Strength or Dexterity saving throw (DC 20) or be Grappled by Xe'noth. Creatures grappled in this manner must repeat the saving throw at the start of their turn; on a success they break free and may take their turn as normal, on a failure they suffer 2d12 + 10 Bludgeoning damage and are considered Restrained. 

Gaping Maw. Melee Weapon Attack: +19 to hit, reach 10 ft., 1 target. Hit: 37 (6d8 + 10) Piercing damage. Any Large or smaller creature that is struck by this attack must succeed on a Dexterity saving throw (DC 20) or be swallowed by Xe'noth. A swallowed creature is blinded and restrained; it has total cover against attacks and other effects outside Xe'noth, and it takes 48 (8d8) acid damage at the start of each of Xe'noth's turns. If Xe'noth takes 30 damage or more on a single turn from a creature inside it, it must succeed on a DC 21 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of Xe'noth. If Xe'noth is slain, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20 feet of movement, exiting prone.

Frightful Presence. Each creature of Xe'noth's choice that is within 120 feet of the aberration and aware of it must succeed on a DC 22 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 Xe'noth's Frightful Presence for the next 24 hours.

Maddening Whisper. A creature of Xe'noth's choice that is within 60 feet of it must succeed on a DC 22 Charisma saving throw. On a failed save, the targeted creature is seized by madness and falls prone, incapacitated and speaking in tongues. On a successful save, the effect is negated. At the end of each of its turns, the affected creature may repeat its saving throw. If the affected creature fails three consecutive saves, it must move to and make every effort to attack the nearest creature to it, regardless of target. The target may still attempt saving throws at the end of each turn. Once it has succeeded on a saving throw, that creature is immune to Xe'noth's Maddening Whisper for the next 24 hours.

Xe'noth 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. Xe'noth regains spent legendary actions at the start of its turn.

Attack. Xe'noth makes a single attack.

Spellcasting. Xe'noth casts a spell of 6th level or lower.

Summon Writhing Tentacle (Costs 2 Actions). Xe'noth summons a Writhing Tentacle of Xe'noth in an unoccupied 5 ft. by 5 ft. space within 60 feet that it can see. The Writhing Tentacle occupies its own initiative slot, which is determined when it is summoned.