The Coalition is a triple-alliance (triumvirate) between 3 different races of aliens. They occupy around 70 systems south of the Free Worlds' territory. Their highest ranking people are called Heliarchs.
Before the events of Endless Sky, they were controlled by the Quarg. After they drove them away, they flourished and formed their own government.
After completing 30 random missions for them, you get access to their freighters and transports. Only the Heliarchs have access to combat ships, and the only way you can obtain said combat ships and their weapons are via plundering.