I haven't played since early BFA. But even I gathered, from the monumental amount of clues, that the Jailor was behind the Scourge, and Frostmourne, and it was all his intent to bring Death to reality (or at least Azeroth) and to control. It bothered me so much that Sylvanas would hate the Lich King, and then side with the Lich King's fucking boss... and this, this is the point she realizes "Oh he wants to make people serve like Arthas did to me"? Wow. Alrighty, then. The writers made Sylvanas seem like a complete dunce.