WARNING: The following contains spoilers for Black Clover Chapter #285, “A Hellish Game of Tag,” by Yuki Tabata, Taylor Engel and Annaliese "Ace" Christman, available now in English from Viz Media.
Chapter #285 of Black Clover, “A Hellish Game of Tag," rewinds time to before Nacht suffered a devastating injury from battling two high-end devils. In his moments of desperation, Nacht has a flashback of him and his twin brother. It turns out, the Vice-Captain of the Black Bulls is named Morgen Faust, and his twin brother is actually Nacht Faust.
There is no clear reason why Morgen changed his name to Nacht, or the need for all the secrecy, especially when it comes to his “new” devil powers. However, there are several clues throughout the manga that paint a bigger picture of the two brothers' relationship, as well as their involvement with Yami and potentially, the Dark Triad.
The House of Faust was a noble family, which meant that the members were magically gifted. The birth of the twins, Morgen and Nacht, is pretty much the epitome of being 'two sides of the same coin:' Morgen was a talented magic knight that saved others, while Nacht only used his power for himself. At the time, Morgen’s magic was different to what it is right now, but that doesn’t necessarily mean Morgen developed a new magic style altogether. Something tragic must have happened in the past that connects to the real Nacht's disappearance, not to mention Morgen’s “newly” acquired-devil powers.
In Chapter #263, Jack remembered Morgen from their early days but noticed that Morgen’s magic powers were different. Morgen simply stated that that man no longer exists and brushed off the comment. During the flashback, Yami and Morgen were part of the same Magic Knights team, the Aqua Deer, and were called on a mission. Both of them being close teammates could indicate that Morgen had dark magic, to begin with, like Yami. With that small correlation, a bigger story starts to unfold, one that could explain the reason for Nacht’s disappearance.
Since Nacht was a free soul roaming the land, he could have been confronted by the Dark Triad. Morgen has a deep hatred for unjust evil, as described in Chapter #263, which can also be said of the Dark Triad to a certain extent. In an unfortunate confrontation, the Dark Triad must have killed the original Nacht while Morgen and Yami were away on a mission. This would explain how Morgen acquired his new powers and developed a particular hatred for Yami as well.
Upon finding out that his brother has been murdered, Morgen expresses a tremendous amount of hatred that triggers a Devil Union Ritual. Four devils become his new partners as a result. Aside from that, Morgen probably blames Yami for calling him away to a mission, which evidently left Nacht on his own, leading to the confrontation with the Dark Triad.
Morgen’s current goal in the Black Clover manga is to provide the Magic Knights with enough time to succeed -- he does not care what happens to himself. Morgen was a different person in the past compared to now. Giving away his life for the sake of the mission would be much more impactful if the original Nacht was truly killed by the Dark Triad. The simple fact that Morgen has taken on the name Nacht means that he's trying to honor the memory of his brother and truly loved him. It would be safe to assume that his older twin brother would also want Morgen to live a full and free life, and now, Morgen is ensuring that the new generation of Magic Knights lives on by prioritizing the success of the mission over his life.
Morgen and Nacht’s relationship is still a mystery, but now we know the two did care for each other. Given that Morgen’s home was a complete mess when he took on Asta and Liebe's training, the Faust family lineage must be coming to an end. Therefore, Morgen’s connection to the Dark Triad must stem from the disappearance of Nacht, and the reason he's spying on the Spade Kingdom. The name “Nacht” is the last connection Morgen shares with the family lineage, and it’s going to be the name he takes to the grave.