Magenta threatening Nightshade after the defeat of the Albino, "Brute".
The Pit is a notoriously brutal fight club for off-duty soldiers who wanted to engage in a more aggressive form of paw-to-paw combat training. It was not sanctioned by the Academy, but leadership didn't appear to be in a rush to shut it down.
From the surface, The Pit looks to be little more than a hole in the ground between a pair of rock spires. Rodents often hang out around the spires to show off their scars and injuries. There are tables strewn around the outer edges of the room where suspicious looking mice play cards, throw darts, and give threatening looks to anyone around. Wooden support beams jut up from the dirt floor, giving it a very rustic look.
In the center of the room, there is a sparring floor where many spectators gamble on the fighting rodents. Nightshade calls them "uncivilized brutes." and accidentally picks a fight with three burly mice he names Brute, Tremble, and Fang. Towards the end of the fight, Brute has Nightshade in a strangle hold, but Magenta steps in and saves him. Ziro is extremely impressed by her skills, and goes looking for her to recruit her as lieutenant for the Genesis Squad. He follows Magenta, only to be threatened by her with a knife. Ziro confesses he knows who Magenta was previously; Lieutenant Magenta of the Alpha Squad. However, she realizes that Ziro is Nitro's younger brother. She believes Ziro's offer to join his squad was a joke set up by Nitro, but Ziro convinces her otherwise. In the end, Magenta agrees to join.