This small, squat building is owned by an old goblin named Gorbits. The interior is always dim or dark, and anyone who comes in with a lantern is immediately told to extinguish it. Inside, two dozen roost line the walls and the centre of the room is dominated by a large cage with a number of small wooden perches inside. The roosts are filled with bats—as is the cage, when a fight is in progress.
Gorbits trains fighting bats and pits them against one another, with spectators betting on the outcomes; he takes a cut of all bets. The fights are never to the death, but usually to first blood or which bat is pinned first. Gorbits has been breeding these bats for a decade now, and they are much tougher than typical bats.
He is willing to sell one of these bloodthirsty bats for 50gp, and someone with the Find Familiar spell can use it to magically bond with one. Otherwise, the bat must be trained to recognize its new owner as its master through Wisdom (Animal Handling) checks.