On the season finale of Gotham, Jim Gordon was infected with a virus that was giving him extreme strength and violent tendencies. And he wasn't alone. The entire city was reeling from the virus and every villain in town knew that whoever had the antidote, had the power.
Though Oswald and Fish Mooney (played by Jada Pinkett Smith) have had a rocky relationship which includes Oswald killing her (she came back to life), they put their differences aside and banded together to get the antidote from Professor Hugo Strange. Oswald used some tortuous tactics to get Strange to take him and Fish straight to the antidote where they found enough to cure every infected person in Gotham.
But Fish wasn't the only one who wanted it. There were some fancy ninjas who wanted it too. Those ninjas used their moves to cut them off from the exit, but things got even more crazy when Jim Gordon entered the scene after tracking Fish down. Gordon quickly became a super strong samurai version of himself, cutting down ninjas left and right. But when his sword filleted Fish, it was unexpected and completely out of character.
Though Fish has died and come back before, she said this was it. And as the city tried to unravel itself out of chaos, Oswald and fans of Gotham said goodbye to Fish Mooney, a beloved Gotham villain.