OBI-WAN:

If you strike me down, I shall become more powerful than you can possibly imagine.

DARTH VADER:

(hoooo… haaaaaa… hoooo….. haaaaa) Are you sure, Old Man?

OBI-WAN:

Why, of course, Darth. Strike me down and…

DARTH VADER:

(hoooo… haaaaaa) It’s just… well… my Master can shoot lightning from his hands.

OBI-WAN:

Ah, well, quite. No, I mean a whole different kind of power, a power greater than anything you…

DARTH VADER:

And (hoooo haaaaaaa) he controls the entire Galaxy. He’s the Emperor of it.

OBS-WAN:

Ha! A mere trifle, my former pupil. You can’t imagine how powerful I will become if you strike me down. Really, really powerful.

DARTH VADER:

It’s just (hooo haaaa) I can imagine a lot of power, old man. If I strike you down, which, by the way, I’m definitely going to do, you’ll become a corpse.

OBI-WAN:

No Darth. I will become a spooky voice over, able to transmit my thoughts into the mind of a young boy who’s coming to destroy you. The force is strong with that one!

DARTH VADER:

(hoooo haaaaa hoooo haaaaa hoooo haaaaa) I see. I take it the rebellion aren’t able to stretch to long range radios then? You need to throw yourself at the mercy of a Sith Lord and get your ass spanked into oblivion in order to communicate helpful advice to your apprentice?

OBI-WAN:

When you put it like that.

DARTH VADER:

You didn’t think this one through did you? (hooo haaa) You are a fool, Old Man!

OBI-WAN:

Ah, but who is the greater fool? The fool, or the fool who follows him?

(DARTH VADER strikes down OBI-WAN who does not, in fact, at any point, become more powerful than DARTH VADER can possibly imagine)

With thanks to @lingmops