As revealed in the books, The Crimson King is the mastermind behind the destruction of the Dark Tower which Roland works so tirelessly to save.
Though through much of the story The Crimson King exists more as an idea than an actual character, his desire to rule over the ashes of all realities is what spurs the conflicts of The Dark Tower.
McConaughey offered a tease of this ultimate evil on his Twitter account today, revealing clearly for the first time the sigil of The Crimson King.
Like Saruman was to Sauron, The Man in Black serves as The Crimson King’s emissary and right hand man, giving form to the evil he represents.