The Coliseum.
The Coliseum (闘技場, Koroshiamu) is a popular venue for underworld arena-style matches. It is located on a remote island that isn't marked on any maps, and therefore can only be found by those who already know where it is. The island has a number of upscale buildings that offer different (and often illegal) goods and services to criminals and those with more money than morals, but the Coliseum is the island's main attraction.
Similar to the Chūnin Exams, the Coliseum's combatants fight each other in one-on-one matches in front of a crowd. Unlike the Chūnin Exams, the combatants are rarely willing participants, instead being forced to do battle by the island's wealthier patrons who have "acquired" them through various means. Although spectators can gamble on the fights' outcome, the greatest gains can be made by the combatants' owners: the losing combatant becomes the property of the winning combatant's master. Fighters with kekkei genkai are considered especially lucrative. Though combatants are typically little better than slaves, some do have genuine attachments to their owners, and as such become deeply upset if their loss in a fight separates them from said owner.
After many years of operating without the notice of the hidden villages, the Coliseum is eventually raided, most of the island's patrons are arrested, and the slaves are set free. In the Sasuke Shinden novel, it is a Kirigakure raid led by Chino; in its anime adaptation, it is a Kumogakure raid led by Sasuke Uchiha.