Darker than Black: Kuro no Keiyakusha, Fate/Zero Recommendations
Action shows with involving stories, dark atmospheres and very pretty, very flashy battles.
Strong main male characters in Hei and Kiritsugu, both distant but powerful. 'Contractors' in Darker Than Black are the main source of supernatural power, whereas in Fate/Zero they come from a third party 'Servants'. Both have plots you can sink your teeth into and have a very similar feel to them, though differing plot.
Both shows set a pretty dark mood. There's death involved, which includes a decent amount of blood, which makes both of these shows pretty serious in comparison to shows that only have people getting beat up instead of actually dying. Both shows are set in the near future, where not many things have changed, but enough to where it definitely affects the plots of both shows. Both shows have a decent amount of action, and little to no comedy, but the great action and serious tone make for a great plot. So if that's what you're into, then watch it!
Both Anime have relatively dark plots and multiple twists
Hei has a very similar personality to Fate Zero's Emiya Kiritsugu
Both are not afraid to kill off characters
Based around a supernatural phenomena:
Darker than Black- Hell's Gate
Fate Zero- Holy Grail
People have unique abilities/magic and only a very select know about these abilities, results in a lot mutal and innocent deaths
Both the Hei and Kiritsugu are much alike
Both share dark atmosphere
Both animes have great batles and are very sad. Same feelings.