Hush fired Prometheus; Deathstroke's Kobra men captured him. Talia sent them after Hush. He was also captured. Hush was unable to cure him - he'd been infected by one of the Ivy clones. Talia promised him that if Prometheus died, he would, too. Hush got him stabilized and was able to escape.
The key they were looking for (which Talia believed Prometheus knew where to find) was uncovered in the JSA's headquarters.