I was passing through the Jade Starlight Forest grind area (I wasn’t even actively grinding). A huge snowstorm started, and then a minigame began where Erebjork spawned. After a 1–2 minute fight, I killed it. I got Caphras and a bunch of Black Stones.
It also gave me an “S” knowledge entry.

MAYBE there is other few methods to get the knowledge as well...