Large large construct (demon), chaotic evil

Armor Class: 17 (natural armor)

Hit Points: 189 18d10+90

Speed: 40 ft

STR

22 +6

DEX

10 +0

CON

20 +5

INT

5 -3

WIS

12 +1

CHA

7 -2

Saving Throws: Str +10, Con +9

Skills:

Siege Monster.

The infernal deals double damage to objects and structures.  

Fiery Impact (Recharge 5–6).

When the infernal slams into the ground (such as upon being summoned), each creature in a 20-foot radius must make a DC 17 Dexterity saving throw. On a failed save, a creature takes 35 (10d6) fire damage and is knocked prone. On a successful save, the creature takes half as much and isn’t knocked prone.  

Fel Core Detonation (When reduced to 0 HP).

When the infernal is destroyed, it detonates in a burst of fel energy. Each creature within 30 feet must make a DC 17 Constitution saving throw, taking 45 (10d8) fire damage on a failed save, or half as much on a success. The scorched ground becomes difficult terrain until cleared.

Damage Resistances: cold, lightning, thunder; bludgeoning, piercing, and slashing from nonmagical weapons

Damage Immunities: fire, poison

Condition Immunities: charmed, exhaustion, frightened, paralyzed, poisoned

Senses: darkvision 120 ft., passive Perception 11

Languages: Understands Eredun and Infernal but cannot speak

Challenge Rating: 12 ( Chaotic Evil XP)

Proficiency Bonus: 4

Actions

Multiattack. The infernal makes two Slam attacks.   Slam. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 18 (2d10 + 6) bludgeoning damage plus 9 (2d8) fire damage.   Fel Pound (Recharge 5–6). The infernal slams its fists into the earth. All creatures in a 15-foot radius must succeed on a DC 17 Strength saving throw or be knocked prone and pushed 10 feet away. Structures and objects in the area take 40 bludgeoning damage.

Lair Actions

On initiative count 20 (losing initiative ties), the infernal can cause one of the following effects in its lair:   Fel Eruption. A fissure opens at a point within 60 feet, spewing fel flames in a 10-foot radius. All creatures in that area must succeed on a DC 16 Dexterity saving throw or take 22 (4d10) fire damage.   Burning Ground. The ground in a 20-foot square becomes molten for 1 minute. It counts as difficult terrain, and creatures entering or starting their turn there take 2d6 fire damage.