Final answer:
Morgan Faust in 'Black Clover' was killed by his twin brother, Lucifero, during an incident involving forbidden magic that also significantly impacts Nacht, another main character.
Step-by-step explanation:
In the anime and manga series Black Clover, the character Morgan Faust was killed by his twin brother, Lucifero. The backstory of this event unfolds in the Spade Kingdom arc, where it is revealed through the memories of Nacht, another character in the series. The incident that led to Morgan's death showcases the themes of sacrifice and the consequences of dabbling with forbidden magic.
Morgan was known to be a kind-hearted and just person, who deeply cared for his brother despite Nacht's involvement with dark powers. Unfortunately, Morgan's attempt to save his twin from the dangers of dark magic led to his tragic end, leaving a lasting impact on Nacht and setting up further character development.