Mara Jade infiltrating Jabba’s Palace to kill Luke.
The whole point of ESB and ROTJ is that Vader and the Emperor want to turn Luke to the dark side.
That’s an interesting point that’s never occurred to me before. However, was her mission to specifically kill Luke though? I checked Wookieepedia and it never states that she was sent to kill him. The whole thing about Palpatine telepathically ordering her to kill Luke happened as he was falling to his death, or at least at some point during or after Vader’s betrayal. Though either way it does raise a question about what exactly her purpose at Jabbas palace was.