Pit Fiend

If the monster dies outside the Nine Hells, its body disappears in sulfurous smoke, and it gains a new body instantly, reviving with all its Hit Points somewhere in the Nine Hells.

The monster emanates an aura in a -foot Emanation while it doesn't have the Incapacitated condition.
Wisdom Saving Throw: DC , .
Failure: The target has the Frightened condition until the start of its next turn.
Success: The target is immune to this monster's aura for 24 hours.

The monster has Advantage on saving throws against spells and other magical effects.

If the monster fails a saving throw, it can choose to succeed instead.

The monster makes one attack, two attacks, and one attack.

. . If the target is a creature, it must make the following saving throw.
Constitution Saving Throw: DC .
Failure: The target has the Poisoned condition. While Poisoned, the target can't regain Hit Points and takes damage at the start of each of its turns, and it repeats the save at the end of each of its turns, ending the effect on itself on a success. After 1 minute, it succeeds automatically.

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The pit fiend casts Fireball (level 5 version) twice, requiring no Material components and using Charisma as the spellcasting ability (spell save DC ).. It can replace one Fireball with Hold Monster (level 7 version) or Wall of Fire.