Rise of the Zilaart in Final Fantasy XI? For most American players that's kind of difficult to do; the expansion came baked in with the launch client and more or less formed part of the base game from a player standpoint. It's probably part of the reason the expansion's two main antagonists, Kam'lanaut and Eald'narche, have been quietly forgotten, even while the game's other villains have received updated forms (like the Shadow Lord and Promathia) or still command player respect (Odin and Alexander).
But it's time for the brothers to step back into the spotlight with the introduction of new high-level battlefields, pitting players against bosses in recreations of their original arenas. Rewards are given both for surviving and for downing the boss, including materials to upgrade artifact armor to level 119. So if you've been spoiling for a rematch with all of your character's new skills and items, you're about to get your chance.