Orcus. Also known as the Lord of the Undead is one of the most powerful Demon Lord. He is at war with Demogorgon and Graz'zt for the title of Prince of all Demon.
You do not know much about him other than the fact that he is somehow linked to November. You do not even know the nature of that link.
But you've seen him in action... You've seen him fight! And that forceful battle resulted in the event now called "The third fall of Demogorgon".