Vader revealed that he is Luke's father

In The Empire Strikes Back, Darth Vader reveals to Luke Skywalker during a lightsaber duel on Cloud City that he is, in fact, Luke's father. Shocked and horrified, Luke initially refuses to believe it, exclaiming, "No, that's not true! That's impossible!" Vader urges Luke to join him and rule the galaxy together, but Luke, devastated, lets go of the ledge he’s clinging to, choosing to fall rather than join his father. This moment is a major turning point, reshaping Luke’s understanding of good and evil and his own path forward.