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

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

Wisdom Save: DC 21. target: 20-foot radius, enemy.

The monster makes one Bite attack, two Devilish Claw attacks, and one Fiery Mace attack.

Save Save: DC 21. target: creature. Melee Attack: +14 to hit. reach 5 ft. Hit: 3d6 + 8 piercing damage.

Melee Attack: +14 to hit. reach 10 ft. Hit: 4d8 + 8 necrotic damage.

Melee Attack: +14 to hit. reach 10 ft. Hit: 4d6 + 8 force plus 6d6 fire damage.

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

A bright streak flashes from you to a point you choose within range and then blossoms with a low roar into a fiery explosion. Each creature in a 20-foot-radius Sphere centered on that point makes a Dexterity saving throw, taking 8d6 Fire damage on a failed save or half as much damage on a successful one. Flammable objects in the area that aren't being worn or carried start burning. Using a Higher-Level Spell Slot. The damage increases by 1d6 for each spell slot level above 3.

Choose a creature that you can see within range. The target must succeed on a Wisdom saving throw or have the Paralyzed condition for the duration. At the end of each of its turns, the target repeats the save, ending the spell on itself on a success. Using a Higher-Level Spell Slot. You can target one additional creature for each spell slot level above 5.

You create a wall of fire on a solid surface within range. You can make the wall up to 60 feet long, 20 feet high, and 1 foot thick, or a ringed wall up to 20 feet in diameter, 20 feet high, and 1 foot thick. The wall is opaque and lasts for the duration. When the wall appears, each creature in its area makes a Dexterity saving throw, taking 5d8 Fire damage on a failed save or half as much damage on a successful one. One side of the wall, selected by you when you cast this spell, deals 5d8 Fire damage to each creature that ends its turn within 10 feet of that side or inside the wall. A creature takes the same damage when it enters the wall for the first time on a turn or ends its turn there. The other side of the wall deals no damage. Using a Higher-Level Spell Slot. The damage increases by 1d8 for each spell slot level above 4.