Wererat

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The monster makes two attacks, using Scratch or Hand Crossbow in any combination. It can replace one attack with a Bite attack.

. . If the target is a Humanoid, it is subjected to the following effect.
Constitution Saving Throw: DC .
Failure: The target is cursed. If the cursed target drops to 0 Hit Points, it instead becomes a Wererat under the DM's control and has 10 Hit Points.
Success: The target is immune to this monster's' curse for 24 hours.

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The monster shape-shifts into a Medium rat-humanoid hybrid form or a Small rat, or it returns to its true humanoid form, 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.