The Black Pits is a series of sequentially more difficult arena style battles, spread across three tiers. Characters may reach roughly level 10 (level 9 for some high experience requirement classes and level 11 for some low experience requirement classes, but generally level 10).
One may export a level 10 character just before the end of the final (15th) battle, and then use this character as the starting character for the main game in Baldur's Gate:Enhanced Edition. If one chooses to do this, the player character remains at level 10 (or 9 or 11), but has his/her experienced reduced to the xp cap (161,000) upon starting the game.
Gameplay is roughly 8 hours for the first playthrough, or 2-3 hours if importing a strong character.
There are several characters in the Pits: Baeloth the Entertainer (drow), Thardek the duergar, Gorch the duergar, Magda the duergar, Dingar the Mad (duergar) Bellowgulp Bluefingers (duergar), Xithiss the mind flayer, Elan Garaq the rakshasa, Brendak the duergar, and Devlin Taralon the elf. There is also Ghlouralk the beholder.