Make it "Fadeable" and write a Fadegroup name for it if you want other things to fade if any item in that group fades at the same time (useful for forests, or sides of tall walls with tons of clutter hanging on the wall)
Set opacity to 0. Thought bear in mind the editor is a bit buggy in that regard, I usually have to restart the editor after setting something to being fadeable before it actually fades.
