Boggart

Fear Monger. The boggart automatically learns the worst fear of any creature within 100 feet that the boggart can see. This trait does not function if the creature has an Intelligence score of 4 or lower.

Magic Resistance. The hag has advantage on saving throws against spells and other magical effects.

Innate Spellcasting. At Will: Barkskin, Enlarge/Reduce, Levitate and Polymorph (Self Only ) 1/Day Each: Spider Climb (Self Only), Water Breathing (Self Only)

Change Shape. The boggart magically polymorphs into the worst fear of a creature that it can see within 100 feet or back into its true form. If the creature's worst fear is a creature that has a challenge rating higher than its own, then the boggart cannot use any actions that it gains because of its new form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the boggart's choice).

In a new form, the boggart retains its alignment,  ability to speak, proficiencies,  and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form. If the new form is an inanimate object, then the boggart has a speed of 0.

When the boggart uses this action, it can use Radiate Fear as a bonus action.

Claws.(Boggart Form Only). Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) slashing damage.

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 10 (2d6+3) [Piercing] damage. 

Radiate Fear. While the boggart is transformed into a creature's worst fear, the must make a DC 14 Charisma save or be frightened of the boggart for one minute.

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

Legendary Action Name (Costs # Actions). Enter the description for your legendary action.