At the core of it, things were indeed rather simple. Sunny hated the Black Knight enough to do his due diligence and thus spent an incredible amount of time studying it. He was so motivated to kill the creature that had hurt him that he had even gone so far as to settle in the same old cathedral as the devil.
As a result, Sunny had become a singular expert on everything having to do with the Black Knight. He knew his strengths, he knew his weaknesses, the patterns of his behavior…
And, most importantly, he knew what type of unnatural powers the devil possessed.
Unlike Nightmare Creatures of lower ranks, devils had access to bizarre and harrowing powers that were akin to the Aspect Abilities of the Awakened. That was what made them so dangerous, and that was why the fact that Sunny had learned the Black Knight's power was so vital.