system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] What if Luke had accepted Vader's offer?

"Join me and we will rule the galaxy as father and son!"

"...ehhh...alright. Lets do it."

"Come with me, it is the only...wait seriously?"

"Yeah I got nothing else going on. Do I get an allowance?"

"We'll talk about this later."
gpt
If that:

Luke begins his sith training, but his obvious corruption leads Vader to turn against the Emperor immediately, since there can only be two. Sidious is murdered by Vader, who becomes the new emperor, and Luke is his new apprentice. 

Han Solo is turned over to Jabba as expected. Without Luke, the Alliance cannot mount a rescue, and their best pilot is lost. However, Han's piloting is never relevant after his capture, and he in fact plays a minor role on Endor - minor directly, that is. He remains trapped in carbonite but he doesn't affect the outcome of the battle. Chewbacca probably dies trying to rescue him.

Lando still joins with the Alliance and leads the strike against the second death star.

Leia is never discovered to be Luke's sister, because the dark side prevents Luke from utilizing the familial bonds of love between the two of them. Her performance during the battle on the forest moon is diminished because she has lost her brother and the man she loves. 

Since Luke turns before the Rebel plan against the second death star is revealed to him, he can't interfere with the attack. **But** both Luke and Vader were excellent pilots, so they may be able to stop the attack. In fact, Luke, having already destroyed a death star, is probably the person most capable to defend one. He and Vader each pilot ships and destroy numerous Rebel ships, including the Millennium Falcon. 

With Leia's underperformance in battle after losing her (unknown) brother and her love, the Rebels fail to bring down the shields, while Luke and Vader destroy the most important battleships of the Alliance. The second death star is completed and it promptly destroys the forest moon, along with Chewbacca, Leia, and the droids.

Most of the Rebel Alliance is destroyed. Scattered rebels try to regroup, but to no avail. More death stars are constructed, making the entire galaxy too terrified to ever rebel again.

In his retirement years, Vader captures Jar Jar Binks and tortures him with force lightning for a century.