Why is the Son from the Mortis trilogy on the Star Wars Rebels poster? Isn't he dead?
That is not the Son from Mortis trilogy, but a (admittedly similar looking) new Dark-side Imperial character called the Inquisitor.
From the wiki:
The Inquisitor was a male Pau'an agent of the Galactic Empire who hunted and eliminated Jedi who had survived the destruction of the Jedi Order. The Inquisitor received his orders from Darth Vader, who tasked him with converting or destroying Force-sensitive children, as well as Jedi who could train them, before they could become a threat to the Empire. To accomplish his tasks, the Inquisitor was trained in the ways of the dark side of the Force and studied the records of the Jedi Temple, giving him insight into the Jedi and their fighting style. As a result, he was a deadly foe for any surviving Jedi.