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

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

Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 5 (2d4 + 1) piercing damage. 

Acid Spit (Recharge 5-6). The spews out a 5ft cone from it's mouth, all creature's in the area must make a DC 12 Dexterity saving throw, taking 6 (3d4) acid damage on a failed save, or half as much damage on a successful save. If a structure is hit with this attack, it becomes vulnerable to all damage until a creature uses an action to clean it off.