The Black Pits II, officially called the Black Pits II: Gladiators of Thay, is a follow-up to Baeloth's Black Pits I. The Black Pits II shows up in Baldur's Gate II: Enhanced Edition. It is run by a powerful mage named Dennaton, enforced by a powerful planetar called The Winged, and staffed by many volunteers (servants?) of Dennaton.
There are several differences between this and the Black Pits I and II. First, the Black Pits II area is much larger than the Black Pits I. You can ask people to help you in battles. There are much more powerful monsters to fight in the arena, because you start at levels 9-11, depending on your class. Dennaton considers you his guests, though you were taken by force.
When you start out, you are in a tavern and a bard is telling the story of your escape from the Black Pits I. She will engage in dialogue, and, no matter what you say to her line, she and the other patrons in the tavern will turn into hostile people, complete with The Winged, an Iron or Adamantite Golem, and an Efreeti. Before this happens, Najim will arrive and warn you that you are in danger. He will be trapped by a magical sphere. Once your whole party is defeated, you will be transported to the Pits. In the unlikely and nearly impossible event (Unless you play with an already trained and well-equipped party) that you win this fight, a dialogue will initiate with Najim, and several options are possible. He will speak about a divination spell to give you a preview of what would have happened had you lost the fight. Agreeing to this will allow you to play the Black Pits II just as if you lost or let them take you by force, while refusing will simply get you back to title screen.