Azrathys, The Keeper of Truth (Ice Devil Variant)

Diabolical Restoration. If the devil 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.

Magic Resistance. The devil has Advantage on saving throws against spells and other magical effects.

Legendary Resistance (3/day). The devil fails a saving throw, it can choose to succeed instead.

Torment of Sins (Recharges after a Short or Long Rest). If Azrathys would be reduced to 0 hit points, its current hit point total instead resets to 175 hit points, and it automatically casts its Mythical Ability Reflection of Sins.

Multiattack. The devil makes three Ice Spear attacks. It can replace one attack with a Tail attack or the casting of Cone of Cold or Ice Storm

Ice Spear. Melee or Ranged Attack Roll: +10, reach 5 ft. or range 30/120 ft. Hit: 14 (2d8 + 5) Piercing damage plus 10 (3d6) Cold damage. Until the end of its next turn, the target can’t take a Bonus Action or Reaction, its Speed decreases by 10 feet, and it can move or take one action on its turn, not both. Hit or Miss: The spear magically returns to the devil’s hand immediately after a ranged attack.

Tail. Melee Attack Roll: +10, reach 10 ft. Hit: 15 (3d6 + 5) Bludgeoning damage plus 18 (4d8) Cold damage.

Ice Wall (Recharge 6). The devil casts Wall of Ice (level 8 version), requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 17).

The devil can take three 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. The devil regains spent legendary actions at the start of its turn.

Attack. The devil makes a bite, ice spear, tail, or frost ray attack.

Truth’s Harrowing Gaze: Forces players to make a Wisdom saving Throw - DC 18. On a failure, they are compelled to speak a painful truth or secret (determined by the DM or drawn from a backstory). The revelation deals 6d8 psychic damage and imposes the Frightened condition for 1 minute.

Ice Step. Azrathys teleports up to 30 feet to an icy surface it can see. This movement doesn't provoke opportunity attacks.

Reflection of Sins: Azrathys manifests illusory duplicates of up to 2 enemies, created from their darkest regrets or sins. These Sin Reflections use the target's stat block with half HP and deal psychic damage instead of their normal damage types. They vanish after 1 minute or when defeated. The target must make a DC 18 Charisma save or be frightened by the duplicate for 1 round.

For this campaign, the enemies will be Brughor and Kalarel.