I assume it was a case of Warwick and Soraka, where he feigned niceness to get her close then used her ult to escape he had the chance. https://pbs.twimg.com/media/DwZq0giWkAUkFgG.jpg
Story says he escaped the hangmans' noose not a prison like in the splash. Splash art isn't necessarily 100% accurate it just tries to convey his mood. Sylas is free he's pissed and Demacians should be very afraid would be that mood.
Everyone else has already said about how he actually escaped the hangman’s noose, I just want to point out something else, which is that he probably legitimately considers Lux a friend, or at least a potential ally. She was kind to him when all of the rest of Demacia hated him and she provided him with his means of escape, albeit unknowingly. Sylas isn’t a bad guy.
The bio is out and this is pretty much what happened. Though it didn't happen in his prison cell.
Looks like Vi.
It's a continuation of the cliffhanger in the trailer
