Advanced Telepathy. Flumphy can perceive the content of any telepathic communication used within 120 feet of it, and it can’t be surprised by creatures with any form of telepathy.

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

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

Prone Deficiency. If Flumphy is knocked prone, roll a die. On an odd result, Flumphy lands upside-down and is incapacitated. At the end of each of its turns, Flumphy can make a DC 10 Dexterity saving throw, righting itself and ending the incapacitated condition if it succeeds.

Telepathic Shroud. Flumphy is immune to any effect that would sense its emotions or read its thoughts, as well as all divination spells.

Multiattack. Flumphy makes two Tendrils attacks or two Fire Ray attacks.

Tendrils. Melee Weapon Attack: +11 to hit, reach 20 ft., one creature. Hit: 22 (3d10 + 6) force damage plus 13 (3d8) acid damage. At the end of each of its turns, the target must make a DC 18 Constitution saving throw, taking 13 (3d8) acid damage on a failure or ending the recurring acid damage on a success.

Fire Ray. Ranged Spell Attack: +9 to hit, range 150 ft., one target. Hit: 18 (4d8) fire damage. If the target is a flammable object that isn't being worn or carried, it also catches fire.

Infernal Spray (Recharge 5–6) Each creature in a 30-foot cone originating from Flumphy must succeed on a DC 18 Dexterity saving throw. On a failed save, the target takes 36 (8d8) acid damage and is coated in a foul-smelling liquid. On a successful save, it takes half as much damage and isn't coated. A coated creature exudes a horrible stench for 1d4 hours. The coated creature is poisoned as long as the stench lasts, and other creatures are poisoned while within 10 feet of the coated creature. A creature can remove the stench on itself by using a short rest to bathe in water, alcohol, or vinegar.

Spellcasting. Flumphy casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 17):

At will: detect thoughts
3/day each: bestow curse, darkness

Summon Servant (1/Day). Flumphy magically summons a hezrou (see the Monster Manual). The summoned creature appears in an unoccupied space within 60 feet of its summoner, acts as an ally of its summoner, and can't summon other demons. It remains for 10 minutes, until it or Flumphy dies, or until Flumphy dismisses it as an action.

Fiery Rebuke. When Flumphy takes damage from a creature it can see within 60 feet of it, Flumphy can force that creature to make a DC 17 Dexterity saving throw. On a failed save, the creature takes 11 (2d10) fire damage.

Flumphy 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. Flumphy regains spent legendary actions at the start of its turn.

Attack. Flumphy makes one Tendrils attack or one Fire Ray attack.

Move. Flumphy moves up to its speed.

Infernal Feeding (Costs 2 Actions). Each creature coated by Flumphy’s Infernal Spray within 60 feet of it takes 22 (4d10) necrotic damage, and Flumphy gains 11 temporary hit points.