As a bonus action, the player can choose to make a double, splitting HP between the two hosts evenly (100hp full body would end up as 50hp in each small body). 

The player can choose to undo the double at any time as a bonus action. When this happens, the user can choose which body they want to make the "host body" and make the double be dragged to the host body at 30ft/round. If the double hits anything/anyone on the way to the host body, the host and whatever is hit takes 2d4 bludgeoning damage. 

Demon Mice are ageless creatures.

Your Dexterity score increases by 2

Your size is Tiny.

You have resistance to fire damage.

You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.

Your base movement speed is 15ft walking and 5ft burrowing.

You have proficiency with Stealth checks.

You get a +2 to your AC