Sinestro is disappointed with guardians, for wanting peace, but always in conflict and never doing what is necessary to end conflicts in the galaxy. For this reason, he built a new troop of lanterns, intended to be a threat to the entire universe, to stop them it will be necessary to unite all troops of different colors to stop the dictatorial Sinestro.
Post-credit scene:
Sinestro decides to unite if there is someone with similar goals to yours and who is capable of bringing fear to the heart of any being ... Darkseide