Multiattack

The harbinger of wrath makes two slam attacks.

Slam

Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 36 (6d8 + 9) bludgeoning damage. The target is grappled (escape DC 20) if it is a Large or smaller creature and the harbinger doesn't have two other creatures grappled.

Impale

The harbinger makes one slam attack against a creature it is grappling. If the attack hits, the target is impaled on the harbinger's spikes. While impaled, the creature is restrained and takes 21 (6d6) piercing damage at the start of each of the harbinger's turns. A creature, including the impaled target, can take its action to free the impaled target by succeeding on a DC 20 Strength check. A freed creature falls prone in a space within 10 feet of the harbinger. If the harbinger dies, a creature is no longer restrained and can escape from the harbinger's spikes by using 10 feet of movement.

Drain Life (Recharge 5-6)

The harbinger drains the life force of one creature impaled on its spikes. The target must succeed on a DC 20 Constitution saving throw or take 55 (10d10) necrotic damage. If a creature dies from this attack, its soul is absorbed into the harbinger and can be restored to life only by means of a wish spell. The harbinger then regains hp equal to the necrotic damage dealt.

Spike Volley (Recharge 5-6)

The harbinger launches a volley of adamantine spikes. Each creature within 60 feet of the harbinger must make a DC 20 Dexterity saving throw, taking 42 (12d6) piercing damage on a failed save, or half as much damage on a successful one.