en.wikipedia.org/wiki/Spirit_King
Firstly, it appears that the Spirit King was a bit of a misogynistic, homicidal maniac:
"Back in the original Golden Age of heroes, Roger Romaine began to kill women in the city of Portsmouth, believing that he would thus absorb their spirits and gain command of the dead to use against the living world. Although he was hunted by Mister Terrific, he was eventually banished to Hell by the Spectre, despite Terrific's insistence that the families of Romaine's victims deserved to see him stand trial."
Check that: Mr. Terrific BANISHED HIM TO HELL. Beat that for badassery, Batman.
And as for what happened later:
"Now trapped in Hell, Romaine made a deal with the demon Shatain, gaining the power to possess and use the bodies of others in exchange for giving Shatain the chance to confront the Spectre. Returning to Earth shortly before the annual meeting with the Justice Society and the Justice League on the League's satellite headquarters, the Spirit King subsequently possessed the body of Jay Garrick, the first Flash, and was able to travel to the satellite. Although Mister Terrific deduced his true identity, the villain killed Terrific, using Flash's super speed so nobody could see who the killer was. Eventually discovering the truth, the JSA chased the Spirit King to Earth, but in the ensuing battle, the Spirit King assumed control of Doctor Fate, and, together with Shaitan, almost succeeded in defeating the Spectre. However, the spirit of Mister Terrific returned and disrupted the connection between Romaine and Shaitan, banishing the Spirit King to Hell once again, although Jay was left feeling guilty about his inability to stop the King using him to kill his friend."