Prisoner of the Mirrors. The Fiend can only manifest within a chamber that is fully walled by twelve mirrors, which have been enchanted by a wizard or warlock. The fiend exists within the mirrors, but can leave them to enter the material plane, surrounded that it is surrounded by the mirrors. If all of the mirrors are shattered, the Fiend immediately melts into a pool of mercury.

Not of this plane. The Fiend has resistance to all damage types.

Magical Resistance. The fiend has advantage on saving throws against spells and magical effects.

Reflected Life. Each of the mirrors in a Fiend of the Mirror World's chamber are made of enchanted glass. They have an AC of 12, 25 hit points, vulnerability to force, thunder and bludgeoning damage and immunity to fire, cold, poison, psychic, necrotic, and lightning damage. If one of the mirrors is destroyed, the Fiend's hit point maximum is immediately reduced by 25 and wounds appear on its body.

Pass through Mirrors. The fiend can enter any mirror and exit through any mirror in which the mirror is reflected. If there are no mirrors reflected around it, the Fiend is destroyed.

Multiattack. The Fiend makes three attacks with its claws, one of which can be replaced by its Drag Me To Hell attack against a target that it has grappled.

Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., 1 target. Hit: 9 (1d8 + 4) slashing damage plus 14 (4d6) necrotic damage and the creature must succeed on a DC17 Strength saving throw or be grappled by the Fiend. Only one creature can be grappled this way at a time.

Drag Me to Hell. The Fiend moves up to its half movement speed towards a mirror that it can see, dragging a creature it has grappled with it. If it enters a mirror, that creature suffers 8d6 psychic damage and is trapped within the mirror-realm and is incapacitated. At the beginning of its next turn the creature may make DC17 Intelligence saving throw to end this effect, or suffers 4d6 points of psychic damage. If the mirror in which the creature is trapped is broken, the creature suffers 8d6 points of psychic damage and falls back into the material plane prone.