Gangplank and the Harrowing?
"Tides of dead will herald my return!"
Is it possible that Gangplank will try to use the upcoming harrowing in his revenge? He could just be saying that he'll kill a lot of people as he returns, but considering how they have an annual zombie apocalypse sort of thing I don't see why he isn't being literal.
I'll burn it all down if I can't have it