He's the company president, but his assistant is 'the (real) power behind the throne'.
In Shakespeare's play, Macbeth kills the king and 'ascends to the throne'.
He 'ascended the throne' [=became king] after the death of his father.
(..) I don't think it's our purpose to understand. Except one thing. We're soldiers. We have to know what we're fighting for. I'm not fighting so some man or woman I barely know can sit in a throne made of swords. So what are you fighting for? Life. Death is the enemy. The first enemy. And the last. (..)2017 TvShow: Game of Thrones Title: Beyond the Wall Season: 7 Episode 6
(..) - I remember. - Father has been charged with treason. He conspired with Robert's brothers against my beloved Joffrey and tried to steal his throne. The Lannisters are treating me well and providing me with every comfort. I beg you, come to King's Landing, swear fealty to King Joffrey (..)2017 TvShow: Game of Thrones Title: Beyond the Wall Season: 7 Episode 6