Go to the Shrine of the Dead (-11, 12) and use Jax's Rocket Launcher to enter it. Go to (-11, 13) and gamble your Koins in order to unlock the Kamidogu and win other items in the process. To obtain the Kamidogu, you must gamble at least 10,000 Koins. Use the Kamidogu to get access to the NetherRealm through the Nether Gate (-24, 18).