Corrupted Rat Queen (Sheila)

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

Advanced Pack Tactics. The rat has advantage on any attack roll or strength check 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.

Corrupted Fortitude. If damage reduces the rat to 0 hit points, it must make a Constitution saving throw with a DC equal to the damage taken, unless the damage is from a critical hit. On a success, the rat drops to 1 hit point instead.

Acid filled Pustules. if above half health and struck in melee, the attacker is struck by acid spurt out from the wound. 3d6 acid damage, DC 15 reflex save for half. 

Multiattack. The rat makes one claw attack, one bite attack, Summon Ratlings and/or Crush but can only use crush action if "Summon Ratlings" is unavailable.

Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 9 (1d10 + 4) slashing damage plus (1d6) poison damage.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 15 (2d10 + 4) piercing damage plus (1d8) poison damage (save for half).

Crush. Any number of creatures within 5 feet must make a DC 15 Dex save or take 23 (3d12 + 4) bludgeoning damage and be knocked prone. If the save is successful the creature takes half damage and is not knocked prone.

Summon Ratlings. Recharge 1d6 - must roll 5-6. - Summon  1d6 "Rats (See Giant Rat)" and 1 Bullrat in the closest adjacent squares. They act following this creatures turn. 

Bursting Carcass. If the rat is killed, or is unable to escape, while at 10 or fewer hit points, the rat explodes and is destroyed. Each creature within 15 feet of it must make a DC 15 Dex saving throw, taking 21 (6d6) piercing damage or half as much damage on a successful save. Any creature damaged this way must also pass a DC 13 Con saving throw, taking 18 (4d8) poison damage on a failed save, or half as much damage on a successful one. If any of these creatures fail this Con save by 5 or more, they are also poisoned for 1 hour. While poisoned this way, a creature is unable to regain hit points.