Incorporeal Movement. The Wraithgoblin Abomination can move through other creatures and objects as if they were difficult terrain. It takes 10 (3d6) force damage if it ends its turn inside an object.

Spectral Resilience. The Wraithgoblin Abomination has advantage on saving throws against spells and other magical effects.

Aura of Corruption. The Wraithgoblin Abomination exudes an aura of corruption in a 40-foot radius. Any creature that starts its turn in the aura takes 10 (3d6) necrotic damage.

Multiattack: The Wraithgoblin Abomination uses its spectral scream. It then makes three spectral claw attacks.

Spectral Claw: Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 30 (4d10 + 7) necrotic damage. If the target is a creature, it must succeed on a DC 18 Constitution saving throw or have its maximum hit points reduced by an amount equal to the damage taken. This reduction lasts until the creature finishes a long rest. If the creature's maximum hit points are reduced to 0 by this effect, it dies, and its spirit becomes a Shade in 24 hours unless it is restored to its original maximum hit points before then.

Spectral Whirlwind (Recharge 5-6): The Wraithgoblin Abomination spins rapidly, creating a whirlwind of spectral energy around itself. Each creature within 20 feet of the abomination must make a DC 18 Dexterity saving throw, taking 44 (8d10) force damage on a failed save, or half as much damage on a successful one. A creature that fails the saving throw is also pushed 20 feet away from the abomination and knocked prone.

Spectral Scream: As an action, the Wraithgoblin Abomination emits an eerie scream. Each creature within 30 feet of the abomination must make a DC 18 Wisdom saving throw. On a failed save, a creature is stunned until the end of its next turn. On a successful save, the creature is immune to the Spectral Scream of the same Wraithgoblin Abomination for 24 hours.

Enter the description for how bonus actions work for your monster here.

The Wraithgoblin Abomination can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The Wraithgoblin Abomination regains spent legendary actions at the start of its turn.

Spectral Swipe. The Wraithgoblin Abomination makes one spectral claw attack.

Spectral Intangibility (Costs 2 Legendary Actions). The Wraithgoblin Abomination temporarily shifts its spectral essence, gaining resistance to all damage except radiant and psychic damage until the start of its next turn.

Spectral Scream (Costs 2 Legendary Actions). The Wraithgoblin Abomination emits an eerie scream. Each creature within 30 feet of the abomination must make a DC 18 Wisdom saving throw. On a failed save, a creature is stunned until the end of its next turn. On a successful save, the creature is immune to the Spectral Scream of the same Wraithgoblin Abomination for 24 hours.