Looked around this section of the forum and didnt see this game mentioned anywhere. This is basically a free and stand alone remake of the first S.T.A.L.K.E.R game; Shadow of Chernobyl, but it has been greatly expanded and new game mechanics introduced from the later games in the series. Well worth a look in my view. You can find it here: https://www.moddb.com/mods/lost-alpha And below, here is a trailer