Abnegazar (left), Ghast and Rath (bottom right)
Abnegazar, Ghast and Rath (the Demons Three) are three demon brothers that billions of years ago ruled the galaxy. However, beings called Timeless Ones banished the demons. However, the demons had created three magic items (the Red Jar of Calythos, the Silver Wheel of Nyorlath and the Green Bell of Uthool) to free themselves. Felix Faust tried to get the JLA to find the items so he could free the demons. Felix accidentally made it so the demon would be freed a century later. JLA, who were time travelling, imprisoned them again. Abnegazar was killed by Doctor Fate. However, his brothers resurrected him. The Demon Three teamed up with Neron (a Devil-stand in). In Infinite Crisis, they are members of the Secret Society of Super Villains and helped Felix Faust in kidnapping Nightshade. The Red Jar, Silver Wheel and Green Bell had cameos in 52 and Teen Titans. They tried (but failed) to use the souls of the magic-users Zachary Zatara, Black Alice and Traci 13 to get out of Hell.
The demons are skilled users of malevolent magic. Because they are magic users, the writer have their powers be poorly-defined and depend on the plot.