The event the black figure was talking about was the Red Rose Day from 5 years ago. It is a night many consider the most iconic in the history of the whole Hitman Guild.
It was the same night that Mikayla saw with his own eyes how much of a monster Kai was. More than that, it was on that night that every hitman recognized Kai as a freakish nightmare of an existence that shouldn't be messed with.
Before, he was indeed a mysterious, intimidating presence in the Guild. But, nobody had yet to see his power so they only saw him as a great source of rumors and perhaps a good fighter with a mysterious background that invented the new Agent number '0'.
However, for better or worse, foolish idiots would still challenge the Black Phantom every single year, hoping of claiming that sweet 'Agent 0' number. Predictably, they all ended up with a bullet in their heads or in an even worse state depending on how tenacious they were.