So Yoda sends Luke off to do battle against Vader and the Emperor, but apparently doesn't even tell him that, although he may defeat Vader, the Emperor is going to microwave him with lightning from his fingertips. All Yoda says is, "Do not underestimate the powers of the Emperor..." but that hardly seems enough! Basically, when Yoda sends Luke off to do this, he is sending him to his death. "Your feeble skills are no match for the power of the darkside." - The Emperor.