Carnifex

Immutable Form. The Carnifex is immune to any spell or effect that would alter its form.

Dimensional Lock. The Carnifex is immune to any effect which would remove it from the material plane.

Built For the End Times. Attacks made against the Carnifex by creatures of a type other than humanoid have disadvantage.

Magic Weapons. The Carnifex's weapon attacks are magical.

Multiattack. The Carnifex makes 2 claw attacks and uses its purification beam if available. 

Claw. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 14 (2d6 + 7) slashing damage and 7 (2d6) fire damage.

Purification Beam (5-6). The Carnifex targets one creature it can see within 60 feet of it. The target must make a DC 16 Dexterity saving throw, taking 18 (4d8) each radiant and fire damage on a failed save, or half as much damage on a successful one.

Active Camouflage. As a reaction to being attacked, the Carnifex can briefly change its appearance to perfectly blend in with its surroundings, causing the triggering attack to miss.