What would of happened........

If Kyp Durron had killed Han and Lando instead of listening to them?

Then Exar may have made a triumphant return. Possibly by taking advantage of Luke's anger that his failed student killed his two friends.

Exar would have turned Streen and someother Jedi to the dark side and recreated the brotherhood of darkness also kyp would have destroyed the empire.

Wrong era

Sorry i ment Brotherhood of the Sith.

There ya go, answered your own question.