Chaos Monster (Doom): Infernal Hell Guard

Legendary Resistance (3/Day). If the Hell Guard fails a saving throw, it can choose to succeed instead.

Infernal Legacy. Infernal creatures ignore a target's resistance to Fire or Necrotic damage

Infernal Lust. All Infernals have advantage against any creature missing any of their Hit Points.

Infernal Protection. Infernal creatures have advantage on saving throws against spells and other magical effects.

Infernal Rite. All Infernal weapon attacks are considered magical.

Infernal Destruction. The Hell Guard deals double damage to objects and structures.

Infernal Terror. Any enemy of the Infernal Hell Guard that starts its turn within 30 feet of it must succeed on a DC 26 Wisdom saving throw or be frightened until the end of their next turn. If the enemy’s saving throw is successful, it is immune to this colossus’s Towering Terror for the next 24 hours.

Infernal Bash. Melee Weapon Attack: +18 to hit, reach 10ft., one target. Hit: 43 (5d12 + 10) bludgeoning damage and the target is pushed up to 30ft, knocked prone. 

Infernal Turret. Ranged Weapon Attack: +18 to hit, range 150/300ft., # target. Hit: 37 (6d8 + 10) force damage and if the target is a creature, it is knocked prone

Infernal Judgment. The Infernal Hell Guard fires a beam of darkness in a 150ft cone. Each creature in the cone must make a DC 26 Dexterity saving throw, taking 75 (10d10+20) force damage on a failed save, or half as much damage on a successful one. If a creature is reduced to 0 hit points by this beam is disintegrated, leaving behind anything it was wearing or carrying. Recharge 5-6 (1d6)

Infernal Mark of Doom. The Infernal Hell Guard chooses a creature it can see within range and mystically marks it as its quarry for 1 minute. Until the mark ends, it deals an extra (1d6) psychic damage to the target whenever it hits with an attack and has advantage on any Wisdom (Perception) or Wisdom (Survival) check it makes to find the marked creature. If the target drops to 0 Hit Points before this effect ends, the knight can use a Bonus Action on a subsequent turn to mark a new creature.

Infernal Fury. After making a melee attack infernal creatures can move up to half their base speed as a Reaction without provoking opportunity attacks.

The Infernal Hell Guard can take 3 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 Infernal Cyberdemon regains spent legendary actions at the start of its turn.

Infernal Bash (Costs 1 Action). 

Infernal Turret (Costs 1 Action).

Infernal Restoration (Costs 2 Action). Can heal for (1d10+20) and end a condition or effect that it is suffering from that can be ended with a saving throw.