The Sewer King ran a child-slavery racket in the sewers of Gotham City.
A man shunned by society and stricken by poverty, the Sewer King lived in the sewers of Gotham City. After residing there for some time, he began kidnapping the orphaned children of Gotham. He manipulated them into thinking that they, like him, were unwelcome in society, and would only find a place in his domain. Giving them a "home", "food", and a "place to rest", he put them to work doing various tasks, including sewing, manual labor, and stealing. After living under such conditions, the children's eyes became acclimated to the darkness. As such, the Sewer King used light to punish them for any offense, such as failure to return home and speaking. Additionally, sewer crocodiles patrolled the area, preventing any unwelcome visitors from straying into the Sewer King's realm.
After a series of thievery reportedly caused by "leprechauns", Batman began investigating the true cause. Cornering one child called "Frog", Batman tried to confront him, but ended up bringing the boy to the Batcave with him. Though the boy refused to speak, he eventually led Batman through the sewers to the Sewer King's hideaway. The Sewer King provided a little difficulty for Batman to apprehend, but he did prevail. Appalled by the villain's actions, Batman expressed a great desire to end the Sewer King's life there and then, but restrained himself and instead handed him over to the police and his crocodiles are caught and sent to the zoo of Gotham.
Abilities and equipment
As far as his persona goes, the Sewer King has no actual abilities. However, he's a master at discipline: though it's unknown how, he tamed several sewer crocodiles (dubbed his "pretties"), and kept several orphaned or runaway children obedient to him under strict rule. As such, he had a small army of crocodiles at his disposal, which he used (amongst other things) to protect his "realm".
The Sewer King's depraved treatment of children caused Batman to consider taking the legal system into his own hands, a rare action for the otherwise calm and collected vigilante.
The Sewer King seemed to have based on two characters from classic children's literature, a portmanteau of Captain Hook from Peter Pan (who actually was scared of alligators instead of controlling them), and Fagin from Oliver Twist, a crook who taught minors how to steal in exchange for their "room and board", whilst he kept a major portion of the loot.