Unlock the mode "Assault" on the map Mount Doom. (Also includes Capture the Ring on Weathertop). Take the One Ring from the Witch King and throw it in the flames of Mount Doom or protect the Witch King who bears the One Ring until the assault ends. This mode looks like it was abandoned by the developers. Works only on the map Mount Doom.
In the folder where the game is installed, go to the "Levels" folder, you'll see
a file named "level_info.dat".
Cut and paste it somewhere else, in a safe place, just in case.
Then, take the custom "level_info.rar" file that you've downloaded, extract it's content and put the new "level_info.dat" file in the "Levels" folder.
Now, in game, on the map "Mount Doom", the mode "Assault" should appear.