Bigger, Better Giant Rat

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

Pack Tactics. The rat has advantage on an attack roll against a creature if at least one of the rat's allies is within 5 feet of the creature and the ally isn't incapacitated.

Rat King. Once per day, the rat can summon a number equal to 1d6 of rat swarms as allies.

Slippery Bastard. The rat has advantage on all strength checks when a grapple is attempted. 

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (2d4 + 2) piercing damage.

Scream. Spell Attack. Each creature within 15 feet of the rat must succeed a DC 13 CON save or become deafened. The save can be repeated at the end of each turn.

Hive Mind. If the rat has 5 HP or less remaining, it can use its reaction to call any allies it may have summoned to cause the attack to miss. This ability can only be used once per long rest.