Under US Copyright law, you cannot protect a short phrase, much less a single word... even an invented word.
You might run into trouble if your Darth showed character traits and behaviors similar to the Darths in the Star Wars novels and films, because then you could be accused pf appropriating the characters.
Aside from copyright laws, you should probably look into trademark. I know that Lucasfilm has trademark protection for Darth Vader in many categories.
Remember that Star Wars is now owned by Disney, which has a reputation for going after "infringers" aggressively. I have no idea if the Lucasfilm legal team has been fully absorbed by the dark side...