Luther: Fallen Sun is - as you might expect, just a longform episode of Luther, but one of the earlier seasons where it wasn't kind of confusing forgettable shit.
I'm pretty sure they start the film with some major retcons to the TV show, but like I said, the later seasons of Luther were shit, so I don't really mind that.
"Fallen Sun" has nothing to do with the plot. Even the unpunned version "Fallen Son" has nothing to do with the plot.
Andy Serkis is a pretty good villain, even if what he does stretches the bounds of credulity into full on super villain territory, but Luther has always existed in a kind of comic book version of London anyway.
The weirdest thing about it is it seems to be a stealth Pilot for a Luther reboot TV show, which you know what? I'm fine with that if Netflix want to splash the cash and keep it as good as early Luther and this film was.