'Worldbreaker <name>', 'Harbinger <name>' or possibly '<name>, Cataclysm's Ruin'... something with a little zest. 'Avenger of the Five Flights' just sounds bland.
Maybe start a list of name they probably wont use. Pretty sure they will not use the following words in the title.
King, defender, twilight, black wing or bane, dragon slayer, vanquisher, Defender of.