Phantasy Star Universe: Ambition of the Illuminus continues the exciting action-RPG storyline of Phantasy Star Universe. A new Human Fundamentalist movement, known as the Illuminus, disrupts the peaceful Gurhal System throwing the Gurhal Government into chaos. Using their custom-made Guardian, players will explore exciting new levels, such as the Ruins of Old Rozenom City and the subterranean Granigs Mines, where they will encounter new and more powerful enemy types, as well as mastering additional versatile class types. Using their custom-made Guardian, players will meet familiar characters like Ethan Waber from Episode 1, and other new characters including GUARDIAN instructor Laia Martinez, whilst they fight through the next chapters of the exciting story missions that started its singleplay experience in the original Phantasy Star Universe.

The expansion further enriches the online Phantasy Star Universe experience with newly created gameplay and customisation features. Players can commune in new and expanded lobby areas filled with colourful decorations and music, including the new Casino Voloyal where roulette and slots can earn players special casino-only items. Character and weapon customisation is even deeper with the introduction of a slew of new weapon types, armour, Photon Arts, and clothing. Each Guardian's individual Rooms are upgraded and now offer a greater level of customisation, with a new grid system that allows players to decorate the walls and place new interactive items and furniture anywhere they want in the Room.