Lucien is a powerful tennis racket with the power to control people. It appears as the main antagonist of Adventure Mode in Mario Tennis Aces.
According to Aster, many years before Mario Tennis Aces, King Bask heard of Lucien's power and took it for himself, only for it to destroy the entire kingdom. However, King Bask managed to strip Lucien of its power and divided the power between five Power Stones, and he hid Lucien in a secret room.
In the beginning of Adventure Mode, Wario and Waluigi, who have got Lucien, give it to Mario and Peach as a "prize" for winning the championships. Despite Toad warning him, Luigi takes the racket, causing him, Wario, and Waluigi to be possessed by Lucien, forcing Mario to go and get the five Power Stones before Lucien does. Toad accompanies Mario in doing this.
Mario and Toad manage to obtain three of the Power Stones, but the possessed Wario and Waluigi take the other two. The group decide to have a tennis tournament in Marina Stadium for the Power Stones. Peach and Daisy need to beat the possessed Wario and Waluigi, while Mario needs to beat the possessed Luigi. After Mario and co. win, Bowser takes Lucien and the five Power Stones for himself, and brings them to the Temple of Bask Shrine. Lucien and Bowser combine to form Bowcien, who then attempts to take down Mario. However, Mario defeats them, breaking Lucien with a Zone Shot in the process. Lucien's remains emit powerful energy which causes the shrine to crumble and nearly cave in on Mario, but he is teleported out safely before he can be crushed by the falling debris.
Afterwards, Mario, Luigi, Peach, Toad, and Daisy celebrated Lucien's destruction, although Wario and Walugi express disappointment in it happening, as they hoped to become the best tennis players with the racket. Daisy then tells them off by reminding them of not only how all the events, including their possession, had been the result of their trying to use Lucien, but that Lucien's powers were ultimately no match to Mario anyway.
Later, Mario and Toad go to the recently appeared Ruins of Trials, where Mario eventually completes the King's Trial (the Last Trial Boss Challenge, where Mario defeats all of the bosses, including Bowcien, in successive rematches) and obtains a non-sentient replica of Lucien as a reward. This replica can then be used by the player when playing as Mario outside Adventure mode.