>>725071829
TL:DR
Sylv made a deal with a new character called "The Jailor" and got a buff so she could go fight the LK.
>Sylv
Longer version.
So they decided to create a new character called "The Jailor" and make him some ultra keikaku thanos rip off who had secretly been behind everything since WC 1, and I mean just about fucking everything. Due to this retcon, WC3 FT inadvertently becomes the Jailor's forces fighting the Jailors forces to...stop himself from achieving his own goal. He was supposed to be more powerful than the previous super satan known as Sargeras, yet he somehow couldn't overpower Nerzhul or Arthas' will when they wore the Helm of domination.
Back to Sylvanas, After WotLK when the LK was killed, Sylv lost all purpose and fucking killed herself by throwing herself off Ice Crown Citadel. Her soul went to super-hell where the Jailor was at, and Jailor offered her a deal to go back and work for him, which she took because superhell is obviously a terrible place. With the offer, she became powerful enough to fight the new LK who took Arthas' place, but bided her time to do several expansions worth of retarded shit first. There's even a really funny event where she basically cucks players who were loyal to her and calls the horde a bunch of niggers before flying off as a fart cloud.