Legacy mode is runing the game on DX9 , how the game always as played in the past.
With DX11 brought with Atlantis, game plays that edition by default you need to select the legacy manually or doing a shortcut , is know to causing some issues on game specially with mods. Since Atlantis changed the funcionality of mods, all previous mods existing are outdated, that measn D2 lith needs to be re imported back to Atlantis, well you need to extract the whole mod files are rebuild again.
Sorry, but sadly its how it is.