This is incredibly epic: a never-before-screened scene cut from Return of the Jedi in which Darth Vader tempts Luke Skywalker to join the dark side, paired with footage of Luke constructing/activating his green lightsaber and officially becoming a Jedi Knight.
As you can see, footage of Darth Vader is spliced with that of his infamous son/rival, as Luke constructs and activates his green lightsaber for the first time—effectively making him a Jedi Knight—in a cave on Tatooine (the first footage of Luke with his Jedi Knight lightsaber in the final version of the movie happens
when he enters Jabba the Hutt's lair to lay the smack down
during the skiff battle).
The footage was apparently screened for the first time today during the "Star Wars Celebration V" convention in Orange County, FL. And while it popped up online this afternoon (it looks to have been shot by an amateur photographer during the screening, who also will forever be embarrassed by his "Yeeeah!" exclamation), it probably won't be there for long. Luckily for you, though, the above clip will be staying put for your viewing enjoyment.
May the force be with you!