Lucien

Lucien is a powerful tennis racket with the power to control people, and appears as the main antagonist of Story Mode in Mario Tennis Aces.

Background
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.

Mario Tennis Aces
In the beginning of Story 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.

However, the possessed Wario and Waluigi manage to take two Power Stones, and decide to have a 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. get the Power Stones back, Bowser takes Lucien, claiming that "it has the power to control people, and it's perfect for a king like him." However, Lucien possesses Bowser, and Mario needs to fight the possessed Bowser to destroy Lucien and complete the game.