Chasm fiend

Siege monster. the chasm fiend does double damage to structures and objects.

impenetrable carapace: any creature that does melee damage to the chasm fiend must succeed on a dc 18 constitution saving throw or be dazed until the start of their next turn. while dazed in this way they lose their reaction, their dexterity bonus to ac, and subsequent attacks while dazed have disadvantage. 

Magical carapace: whenever the chasm fiend takes magic damage it rolls a d6 on a 1-3 nothing happens and it does normal damage, on a 4-5 the magic is reflected at the caster using the casters save dc and to hit bonus, on a 6 it triggers a small magical explosion dealing 5d10 force damage to all creatures within 30 ft except for the chasm fiend.

Vulnerable underside: when prone melee attacks with 5 ft of the chasm fiend auto crit.

Multi attack. The chasm fiend attacks twice with its claws.

Claw. Melee Weapon Attack: +15 to hit, reach 30 ft., 1 target. Hit: 37 (4d12+ 10) bludgeoning damage. On a hit the chasm fiend can forgo damage to grapple the hit creature the creature must succeed on a dc 20 acrobatics(dexterity) or athletics(strength) check, on a fail the creature is restrained until its next turn where it can try to make the saving throw again the chasm fiend has 2 claws that can each hold a creature. when the chasm fiend deals damage with its claws and it has a creature restrained in them the creature takes half of the damage done to the target.