Black Clover's anime adaptation has definitely hit its stride in the second arc, and fans were even recently shown the darkest moment in Black Clover so far. While it has hinted at theme like death (and even featuring one earlier in the series), its nonviolent battles made Mars' origin all the more unsettling.
Mars, the big foe from the neighboring Diamond Kingdom, had been teasing a darker past with flashbacks in his battle with Asta, but no one expected to see him kill his friends as a child.
As noted by Klaus in a previous episode, the Diamond Kingdom is rumored to experiment on its children and embed them with magic stones in order to create the perfect mage. Mars is shown to be one of those children, as he grows closer with a girl named Fana. The two of them believe that they'll be able to go to the outside world (as they've been growing up in some kind of facility) once they become strong mages but this quickly proved to not be the case.
The Diamond Kingdom tells the children to all kill one another, and only the surviving child will be allowed to represent the Diamond Kingdom. Thus, Mars' classmates begin killing one another. Soon enough, the only two left are Mars and Fana. Fana apologizes, but she says Mars needs to die.
She charges at Mars, but Mars' strong magic saves him in the end and pierces Fana. Mars is then chosen as the strongest mage, and is even given some of Fana's magic to make him stronger (by cruelly binding their grimoires together). He later finds out that Fana did this in order to set Mars free, and while this is the darkest Black Clover has ever been it does have a strange silver lining of hope for the future.
If you are unfamiliar with Black Clover, the series debuted in Shonen Jump back in 2015 from creator Yuki Tabata. The since popular series follows two young boys named Asta and Yuno who are growing up in a world where magic is everything. Asta, unfortunately, seems to have been born without magical powers while Yuno is a young prodigy whose magical powers rival that of the strongest magic users in the Clover Kingdom, the Magic Knights. The story then follows the two young boys as they strive to become the strongest wizard in the Clover Kingdom, The Wizard King.
Studio Pierrot's Black Clover adaptation premiered in Japan on October 3 and is currently slated for a hefty 51 episodes. You can currently find the Japanese language version with English subtitles on Crunchyroll, and the English language dub on FunimationNow and Saturday evenings on Adult Swim's Toonami block.