Water Breathing. The Yuinsu can breathe only water.

Innate Spellcasting. The Yuinsu's innate spellcasting ability is Intelligence (save DC 17). It can innately cast the following spells, requiring no material components.

At Will: Detect Thoughts, Magic Mouth, Silent Image
2/Day Each: Calm Emotions, Compulsion, Dream, Enthrall, Hypnotic Pattern

Multiattack. The Yuinsu makes two Tentacle attacks and a Bite attack.

Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 18 (3d8+5) piercing damage.

Tentacle. Melee Weapon Attack: +9 to hit, reach 20 ft., one target. Hit: 12 (2d8+5) bludgeoning damage, and the target is Grappled (escape DC 13). Until this grapple ends, the target is Restrained. The Yuinsu has eight tentacles, each of which can grapple a target.

Phantasms (Recharges after a Short or Long Rest). The Yuinsu magically creates three illusory duplicates of itself. The duplicates move with it, and mimic its actions, shifting position so as to make it impossible to track which Yuinsu is the real one. 

If the Yuinsu is targeted by an attack, roll a d20 to determine whether the attack instead targets one of the duplicates. If it has three duplicates, it must roll a 6 or higher to change the attack's target to a duplicate. With two duplicates, it must roll an 8 or higher. With one duplicate, it must roll an 11 or higher.

A duplicate's AC equals 10 + the Yuinsu's Dexterity modifier. If an attack hits a duplicate, the duplicate is destroyed. A duplicate can be destroyed only by an attack that hits it. It ignores all other damage and effects. The effect ends when all duplicates are destroyed.

Tentacular Impersonation. As an action, the Yuinsu can use a combination of up to eight of its tentacles to replicate the appearance of people or objects. The replicas appear, feel, and sound exactly like the shapes the Yuinsu is impersonating. One tentacle can mimic up to a Medium-sized creature or 2 to mimic a Large creature or object, with 4 tentacles being used to impersonate a Huge creature or object. A DC 17 Intelligence (Investigation) check can notice the thin tentacles below the waters used to form the shapes for this mimicry. The impersonation also fails when exposed to bright light.