Legendary Resistance (3/Summon). If the Fused Shadow Monstrosity fails a saving throw, it can choose to succeed instead.

Magic Resistance. The Fused Shadow Monstrosity has advantage on saving throws against spells and other magical effects.

Magical Being. All of the Fused Shadow Monstrosity attacks are magical.

Hunger for Pain. The Fused Shadow Monstrosity form can only be maintained if it deals damage.  If it has not inflict any damage since its last turn or taken damage since then, the magic holding the form together breaks, separating the 4 pieces again.

Fighting Against Itself. Being in this form awakens the Fused Shadow's Sentience and Personality.  The creature who wears the Fused Shadow must succeed a DC 15 Charisma check every turn, or that turn's actions are dictated by Fused Shadow.

Multiattack. Fused Shadow Monstrosity can use it's Frightful Presence. It then makes three attacks with its arms.

Slam. Melee Weapon Attack: +12 to hit, reach 15ft., one target. Hit: 20 (4d6 + 5) bludgeoning damage.

Clap. Melee Weapon Attack: +8 to hit, reach 10ft., one target. Hit: 20 (4d6 + 5) bludgeoning damage.  On a hit, the target is grappled by Fused Shadow Monstrosity.

Frightful Presence. Each creature within 120 feet of Fused Shadow Monstrosity who is aware of it must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the Fused Shadow Monstrosity is within line of sight, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the Fused Shadow Monstrosity's Frightful Presence for the next 24 hours.

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

Attack. The Fused Shadow Monstrosity makes one claw attack or tail attack.

Move. The Fused Shadow Monstrosity moves up to half its speed.

Twilight Spear. (Cost 3 Actions) The Fused Shadow Monstrosity summons a giant spear made of twilight magic and flings it towards a point.  Every creature within 20 feet of the point must make a Dexterity saving throw.  On a fail, their take 5d6 Cold damage and 5d6 Force damage, on a success, half that.