Amorphous. The Zark can move through a space as narrow as 1 inch wide without squeezing.

Aversion to Fire. If the Zark takes fire damage or is exposed to magical light from a leveled spell, it has disadvantage on attack rolls and ability checks until the end of its next turn.

Unusual Nature. The Zark doesn’t require food, water, or sleep.

Multiattack. The Zark makes two Claw attacks, and it uses Venomous Bite attack.

Claw. Melee Weapon Attack: +7 to hit, reach 15 ft., one target. Hit: 17 (4d6 + 3) slashing damage plus 14 (4d6) necrotic damage.

Venomous Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 10 (2d8) poison damage and must succeed on a DC 18 Constitution saving throw or be poisoned for the next minute. A creature may repeat the saving throw at the end of its turn, ending the effect on itself on a success. 

Innate Spellcasting (Psionics). The Zark casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 18):

At will: detect thoughts

A target of the Zark's detect magic spell must make a Wisdom saving throw versus the spell DC or the Zark learns all of the target's languages, gains all its skill proficiencies, and becomes familiar with the target's most prevalent loved ones.

Enticing Lure. The Zark can use its bonus action to polymorph into a Small, Medium, or Large creature it has seen in a victim's memory, or back into its true form. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies. 

If the Zark is impersonating an entity from a creature's memory, that creature must make a DC 18 Wisdom saving throw the first time they see the Zark in this form or be charmed by it. This charm lasts until the victim takes damage from any source or the Zark changes form.

Slippery Escape. The Zark can use its bonus action to squeeze into the crack of any surface it is in contact with, and travel using its burrowing speed before reemerging. The openings in the surface must be at least 1 inch wide for it to enter or reemerge.  

Terrible Transformation. The Zark targets one creature it can see within 40 feet of it that has just dropped to 0 hit points. The target must succeed on a DC 18 Constitution saving throw or become transformed into a Junior Zark.

A creature transformed into a Junior Zark regains 50 (5d8+10) hit points. The creature acts as directed by the Zark (no action required), and any weapon attacks performed by the creature now deal necrotic damage instead of their original damage type.