Mossador attempts to convince his fellow slaves to join Daenerys Targaryen and defeat the Great Masters, though he meets resistance from skeptical older slaves who have lived through failed slave rebellions. After Grey Worm motivates them even further than Mossador and provides them with weapons, they launch a revolt against the Masters
Mossador, now representing Meereen's freedmen in Daenerys's small council, reports to Daenerys Targaryen along Grey Worm that White Rat, one of the Unsullied, was found with his throat slit at a brothel, murdered by the Sons of the Harpy.
Mossador is present when the council debates what should be done with a captured Harpy's Son. When the queen decides to give the prisoner a fair trial, Mossador interprets Daenerys's decision on the matter to mean that her hands are tied, and without permission, kills the captive. Daenerys, has him brought out in chains before a crowd for execution, as a warning to everyone, freed slave or Great Master, not to pervert the course of justice. He pleads for forgiveness but she has Daario cut off his head, causing a riot.
Formerly a Meereenese slave, Mossador was an advisor to Daenerys Targaryen until he murdered one of her prisoners, a Son of the Harpy. Dany sentenced Mossador to death as punishment for his crime and his public execution triggered a riot between the freedmen and former masters.