The Fergality was a finisher that could only be performed on the Armory stage by Raiden in the Sega Genesis/Mega Drive version of Mortal Kombat II, the defeated player's character turns into Probe Ltd. employee Fergus McGovern, who worked on the port for this game.
First, the version you have to be on of Mortal Kombat II is the Sega Genesis/Mega Drive version. Then, on the main menu screen enter the options menu and highlight 'Done!'. While highlighting, press 'left, down, left, right, down, right, left, left, right, right'. After entering the cheat code, a 'Test Modes' menu will appear, enter it and go to the third page of the menu, then turn the 'Oooh, Nasty!' option on. After that, go on the character select screen and select Raiden and wait until you get to the Armory stage. After defeating your opponent, when the 'Finish him/her!' prompt appears, from any distance enter the input 'back, back, back, block', and the Fergality will be performed.