Lucien

Lucien is an ancient and evil demon of tennis who once terrorized the Kingdom of Bask long ago and sought to achieve absolute power.

Background
Lucien's origins are unknown or how he came to be sealed in the form of a tennis racket. What is known is that long ago in ancient times, Lucien's racket was discovered by King Bask, the ruler of the ancient Kingdom of Bask where tennis prospered. King Bask sought to covet Lucien's power for himself but instead Lucien ended up using to Bask to destroy his own kingdom. In King Bask's last moments, he used what remained of his powers to seal away Lucien's power into the five Power Stones and trap Lucien's racket deep within the Temple of Bask where he hoped it would never harm anyone again.

Mario Tennis Aces
Eons later, the greedy pair of Wario and Waluigi had seemingly heard of this tale and sought the racket's power for themselves, raiding the Temple of Bask in order to find it. However the pair underestimated Lucien's power and it ended up possessing the greedy duo, turning them into its obedient and mindless thralls, using them to make its way into the Bask tennis championships in the hopes of finding a more powerful and worthy host. Initially Lucien tries to have Wario pass the racket onto Mario who rejects it, but is able to convince Luigi to take it, making Luigi the new host to his evil power. With Luigi, Wario and Waluigi under its control, Lucien summons forth an evil storm which wrecks havoc on the stadium, allowing it to make its escape to find its lost Power Stones, and leaving behind a mysterious swirling vortex in the sky.

Lucian's thralls eventually recover all five of the Power Stones, restoring Lucien's power which he proceeds to test out in his newly created Lucien Cup, welcoming all challengers to destruction. However, Mario manages to defeat Lucien regardless and free his victims, but before the heroes can celebrate, Bowser arrives to steal Lucien's racket for himself, giving him absolute power and transforming him into Bowcien, a powerful and proper host for Lucien's power, but unlike its other thralls, Bowser is still conscious, claiming that he and Lucien have become one while reveling in the newfound power. The mighty Bowcien runs off to the Temple of Bask to hone its power but is eventually confronted by Mario who challenges Bowcien to one last match. Bowcien commends Mario's enhanced skills through his training and accepts, still convinced that he will come out victorious. Luckily, Mario is able to defeat Bowcien with a well timed Zone Shot which knocks the tainted racket out of Bowcien's hand and causes it pierce into one of the empowered pillars in the temple, causing the temple to fall apart and destroy Lucien once and for all, although Bowser survives. Later, Mario and all his friends celebrate Lucien's destruction, although Wario and Waluigi express disappointment, having wanted to use Lucien's racket to become the greatest tennis players of all time. Daisy then reprimands the duo, reminding them that everything that happened was because of their own irresponsible actions and reminds them despite all of Lucien's evil power, he was still no match for Mario's skills.

Personality
Lucien is an evil and manipulative being who only cares about spreading destruction, bending all others to his will and proving its superiority in tennis. Because of the latter, he will happily accept any tennis match he's challenged to. Lucien is highly deceitful, as shown by how it manipulated King Bask long ago and how it used Wario and Waluigi as its puppets, even making them compliment their rivals just so they would hold the evil racket and become possessed.

Physical Appearance
Lucien's true form appears as an evil smoke-like mass of shadowy swirling energy with a sinister grimace and sharp eyes that shine an ominous purple glow from within the smoke. When possessing others, his victims turn into shadowy mummy-like revenants of their former selves with stone-like skin and malicious glowing eyes who are wrapped in mysterious evil bandages covered in ancient runes. His victims also emit a dark energy much like that of his true form.

Its racket form appears as a stunning ancient racket with a golden frame with spikes along its sides and its dark purple handle is covered in the same sort of bandage wrappings that his victims are covered with, suggesting that this is where they manifest from. Just above the handle is a special golden racket throat with five slots where the legendary Power Stones must be placed before Lucien can recover its full power. It has five magic Power Stones just below his "sweet spot". When inactive, the racket's spikes are no longer present, shrinking down to small square shapes and its handle appears black and faded. The racket also glows with a sinister dark purple aura when awakened, a manifestation of Lucien's dark power.

Abilities
Despite being confined to a tennis racket, Lucien is a powerful demonic entity as noted by its great power to destroy an entire ancient kingdom and posses anyone who holds his racket, even being capable of possessing multiple beings at once. Lucien is also a masterful tennis player and enhances the skills of anyone he possesses, with even championship announcers acknowledging that a Lucien possessed Luigi had become a vastly more powerful tennis player.

Power Stones
However, all of its great powers were stripped away and sealed by King Bask into the form of five small Power Stones which greatly weakened Lucien's destructive powers. Despite this weakness, Lucien can regain most of his former power by reuniting the stones with his racket and possessing a powerful host such as Bowser, with whom he was able to become the powerful Bowcien. But even without the stones, Lucien is still capable of possessing others and bending their wills to its own as well as causing vile storms that can darken the skies and summon vicious windstorms.

Weakness
For all of its great power, Lucien still needs its Power Stones, but even with them restored, Lucien still has a rather particular weakness in that its power seems to be linked to mysterious ancient pillars in the Temple of Bask which bare glowing green runes reminiscent of the ones on the bandages Lucien uses to possess others, and when Mario knocked Lucien's racket into one of these pillars, it caused his powers to become unstable and was easily defeated.

Trivia

 * Lucien is the second evil entity to manipulate and possess Luigi, the first being Dimentio and the Chaos Heart from Super Paper Mario.
 * His status as an ancient shadow-like purple demon who tried to terrorize the world, was sealed long ago and can transform victims into dark puppets is reminiscent of the Shadow Queen from Paper Mario: The Thousand-Year Door. Both also required the power of enchanted multicolored gems to achieve their goals, with Lucien needing the Power Stones and the Shadow Queen needing the Crystal Stars.