The Dark Knight's Blasphemy Sword!
As a high-level Undead, the Dark Knight, just like other Undead, had the ability to attack the souls of their targets. Their most powerful weapon was the Black Knight's Sword in their hands, condensed from half of their own spirit.
It was the Dark Knight's weapon for devouring the souls of other beings, even the believers of deities would have their souls shattered beneath the sword.
For this reason, the Dark Knight's Knight's Sword was also known as the Blasphemy Sword.
All this information was what Charlotte had learned from the memories that emerged when Sebastian drew his weapon from the void.
Not only that, but she also found out that Sebastian had acquired this Blasphemy Sword during his travels in the Dark Domain when he was still at the Blazing Sun Third Tier after he had slain a Dark Knight.
Such an experience of a Blood Contract was... really quite suitable for gossipmongers.