Carnifex the Toad Demon and Demonic Plumber

5e (simplifed) conversion from Maze of the Blue Medusae for my own reference.

CARNIFEX the DEMONIC PLUMBER

Medium fiend (demon), chaotic evil
AC 18 
HP 170
Speed 30 ft.
Skills Arcana +6, Perception +8
Damage Immunities Fire, Poison
Damage Resistances Chaotic (he is resistant to the first damage type he takes in a round until the end of his next turn).
Condition Immunities Frightened
Senses Blindsight 60 ft., Darkvision 60 ft., Passive Perception 18
Languages Slaad, Telepathy 60 ft.
Challenge 10 (590 XP)

Innate Spellcasting. Carnifex’s innate spellcasting ability is Charisma (spell save DC 15, +7 to hit with spell attacks). He can innately cast the following spells, requiring no material components:

Magic Reflection. Carnifex has advantage on saving throws against spells and other magical effects. If he succeeds at the saving throw due to this advantage the spell is reflected back at the caster.

Magic Weapons. Carnifex’s weapon attacks are magical.

Regeneration. Carnifex regains 10 hit points at the start of its turn if it has at least 1 hit point.

Touch of Chaos. Any physical touch made by Carnifex (bite and claws in combat) invokes his chaotic nature.  He may announce that the target may not do the same thing again.  This curse lasts until Carnifex is dead. The restriction should not be unplayably broad, examples include “Casting a Spell”, and “Attacking with that Weapon”.

ACTIONS
  1. Multiattack. Carnifex makes three attacks: one with its bite and two with its claws or wrench.
  2. Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 9 (1d8 + 5) piercing damage plus 7 (2d6) necrotic damage. He also chucks a vial of auto-corroder at anyone wearing metal armour.  This automatically corrodes the target’s metal equipment. The equipment grates and jams imposing disadvantage until they can be properly maintained.
  3. Claws. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 10 (1d10 + 5) slashing damage plus 7 (2d6) necrotic damage.
  4. Devil’s Wrench. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage plus 7 (2d6) necrotic damage. If used on any reasonably complex machinery, that machine will break down catastrophically after 1d4 further uses.
  5. Cast a spell. Choose a spell, cast it.
  6. Gremlin Bag. Tosses out 1d6+1 gremlins which attack the party.  If left to their own devices will smash machinery.