The folks over at HBO seem to think that there might just be significant overlap between the audience of Game of Thrones and the people who play League of Legends. They probably aren't wrong.
HBO has apparently approached Riot Games to do some sort of advertising deal. By itself, that wouldn't be terribly interesting. All that it means for us common folk is that we'll be seeing some Game of Thrones advertisements on the League of Legends website.
Of more immediate interest, Riot Games is also now working with the HBO team to produce a mash-up trailer of League of Legends and Game of Thrones. What will that look like? We haven't the faintest idea, but we, much like beloved characters in a George R.R. Martin novel, are dying to find out.