Makuta Nightmare was the former tyrant of the league of shadows. He died by an enemy assault where Makuta Cedar didn't show up to help him.
Makuta Nightmare now appears in other's drams and will make any hit in a fight happen to the person in real life. He uses that attack to kill others in their sleep.
Also The League of Shadows are currently trying to kill the resistants in the war and find Primal to revive Makuta Noghtmare.