This show just got darker with the episode 'Monster'. I think it might become darker before it lightens up again.

Supernatural is quite dark at times, though they do know to balance it with some humor. It's got it's own very evil and dark Lucifer who's been quite active as of late. The show has lasted 12 seasons so far. No misprint there BTW TWELVE seasons. Not to shabby don't you agree?

There already are/were many dark series out there, and some of them didn't last long.:)
Lucifer was always more a "light" one, had always a good balance with humour and darker themes and some eps might get more dark eventually but not the whole show.