You will encounter the Ratling Boss in the southwest of the Sewers under the Hamlet of Stormbreach.
He literally carries a big cannon as his weapon and he is surrounded by other ratlings who fight for him.
His cannon does a big knockback, making things more difficult for melee attacks.

The ratling boss and his ratlings have a combined healthpool.

Hitpoints: 1400 + all hitpoints of the other ratlings in the room, which accumulate to another 1400 hp.

Drops: Hand Cannon, Brass Key