The name says it all. It is a section where the Thrall Captains (Urchinfists and Thrall Captains) can be found.
This part of the cavern is inhabited by the highest level of Davy Jones' Crew. This consists of grenade-throwing Urchinfists (Lvl 43-50), as well as Thrall Captains (Lvl 43-50) who wield swords. The entrance is guarded by Seabeards Molusks and other enemies, who also give a difficult challenge. Thrall Captains have powerful cutlass skills and particularly Blade Fury attacks , while the Urchinfists stick to ranged fire.
Don't come here alone unless you're a high-level pirate looking for a good fight. Try and have a crew guarding your back and bring a few tonics!