Assassin Rat

Shapechanger. The wererat can use its action to polymorph into a rat-humanoid hybrid or into a giant rat, or back into its true form which is humanoid. 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.

Keen Smell. The wererat has advantage on Wisdom (Perception) checks that rely on smell.

Assassinate. During its first turn, the assassin has advantage on attack rolls against any creature that hasn't taken a turn. Any hit the assassin scores against a surprised creature is a critical hit.

Sneak Attack. Once per turn, the assassin deals an extra 10 (3d6) damage when it hits a target with a weapon attack and has advantage on the attack roll, or when the target is within 5 feet of an ally of the assassin that isn't incapacitated and the assassin doesn't have disadvantage on the attack roll.

Nimble Escape. The Assassin Rat can dodge, dash or disengage as a bonus action.

Multiattack. The Assassin Rat makes to attacks two dagger attacks or one leap and one dagger attack.

Dagger. Melee Weapon Attack: +6 to hit, reach 5 ft., 1 target. Hit: 3 (1d4 + 3) slashing damage. 

Assassin's Leap.  The assassin rat leaps up to 30 ft in a line at one target. The target then must make a DC 14 Dexterity saving throw. On a failure, the target is grappled and knocked prone beneath the Assassin Rat. On a success, the Assassin Rat stops five feet away from the target.

Smoke Bomb.  The Assassin Rat obscures itself with a smoke bomb and turns invisible until the end of its turn.