The Card Kingdom has a storied history of its former kings, as related by Seam, and the actions of the Knight seem to have taken place a time before the events of Chapter 1, as Jevil not only needed time to meet the Knight, but time to grow insane (and be imprisoned as a result).
The Wiki on the Dark World page treats the "strange someone" Jevil met as the KNIGHT.
Although I do believe this true, I do not recall explicit evidence that this is true.
Additionally, the Wiki varies slightly from saying "The Dark world" to "dark worlds"/"a dark world".
So I think there may be some adjustment required here.
I'm fairly certain that King says that Jevil met the Knight and was the one that told him about them.
But just because Jevil knew the KNIGHT doesn't nessecarily mean the KNIGHT was the strange someone...even if it is almost certain.
King’s dialogue in Castle Town confirms that Jevil has met the Knight, then relayed its brilliance to King, then continues talking about how Seam had to seal Jevil afterwards. Even if the “strange someone” who made Jevil insane was different from the Knight, this dialogue is enough evidence for that statement on the wiki to be true.
Does it? On that page, I only see the term “Dark World”.