The age old question.
Of the two most powerful mages.
Well here's how it happens.
Xerath has finally broken free of his bonds as Veigar approaches him to destroy him, as a threat to Veigar's domination.
Xerath's ascension complete and his power unleashed, he gazes into the heart of runeterra and the secrets of the universe are revealed to him.
Upon this discovery, Xerath simply vanishes, becoming one with the eternal energy flow of the universe, drifting in the infinite knowledge for all of eternity.
Veigar stands there, distracted by the *What just happened?* moment, when Katarina Shunpo's behind him and cuts his tiny yordle throat for breaking out of Noxus's Prison and having a warrant of life or death on him.
And there we have it.