Lux in tenebris
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It's very simple! Did you download the file? Did you put it in the Maps folder, which is located in the game directory? Then, you go into the game, choose not "Single Player" but "Multiplayer"... Then, go to "One Computer", then to "Normal Mode", and look at the list of maps at the top...)):D
Евангелие от Иоанна, 1.5: Lux in tenebris lucet, et tenebrae eam non comprehenderunt. Свет во тьме светит, и тьма не объяла его.