The scene revolves Darth Vader and Luke Skywalker fighting with their lightsabers, when Vader manages to cut off Luke's right hand and forced him to the corner. Vader then revealed that he is, in fact, his father whom Luke was told that had died long time ago. This culminated into Luke screaming and crying out of disbelief, anguish and agony.
Contrary to popular belief, Vader never says the name LUKE during his quote.