Theory regarding Lucian, Senna, The Ruined King and Mordekaiser.

So now that Lucian is set on stopping the big bad Ruined King himself, I think this will be how Morde's past ties to the Shadow Isles come back into play. Something during their fight with the Ruined King will allow Morde to come back to Runeterra, causing him to take over the Shadow Isles and use the Black Mist to fuel his conquest to rule over everything in death.