Copycat

Spider Climb. The Copycat can climb difficult surfaces, including along ceilings, without needing to make an ability check.

Sunlight Sensitivity. While in sunlight, the Copycat has Disadvantage on ability checks and attack rolls.

Unstable. At the end of every round of combat, the Copycat's size category changes by rolling a 1d4. (1: Tiny, 2: Small, 3: Medium, 4: Large). The AC and respective stats fluctuate from the size change.

Multiattack. The Copycat makes two Slam attacks, one Bite attack, and uses Unsettling Visage if available.

Slam. Melee Attack Roll: +7 (with Advantage during the first round of each combat), reach 5 ft. Hit: 13 (2d8 + 4) Bludgeoning damage.

Bite. Melee Attack Roll: +7 (With Advantage during the first round of each combat), reach 5 ft. Hit: (2d4+3) Piercing damage.

Read Thoughts. The copycat casts Detect Thoughts, requiring no spell components and using Charisma as the spellcasting ability (spell save DC 15).

Unsettling Visage (Recharge 5-6). Wisdom Saving Throw: DC 15, each creature in a 15-foot Emanation originating from the Copycat that can see the Copycat. Failure: The target has the Frightened condition and repeats the save at the end of each of its turns, ending the effect on itself on a success. After 1 minute, it succeeds automatically.

Shape-Shift. The Copycat shape-shifts into a Medium or Small Humanoid, or it returns to its true form. Its game statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn’t transformed.

Mirror. (3/Turn). When a player makes an attack targeting the Copycat, it may use one legendary reaction to copy the attack back at the attacker. It will deal the same damage dealt to itself back if it hits. The attack uses the Copycat's own statistics to determine the 'To Hit'. The Damage modifier is unaffected, however.