Xellos the Mysterious Priest is a character from the Slayers series. As the name implies, his origins remained a mystery, thus making the Fantasy Adventure Team uncertain if he's on the good side or on the bad side.
In Benny, Leo, Johnny and Rae Meet the Great Beast, he arrived at the village in Scotland and he married Isla and Lazarus, who is known as the Great Beast, but suspected of Artis' secret love for Lazarus. When he discovered that Artis struck a deal with the Banshee, he "summons" the team to Scotland to expose the Banshee so he could destroy her. However, he failed and was defeated while battling with the Banshee and left to die, but he somehow survived. He was later seen carrying the now human Lazarus to the village for medicine.
His catchphrase when questioned about any sort of knowledge is "that is a secret!".
- Xellos debuts on Benny, Leo, Johnny and Rae Meet the Great Beast.
- Xellos will be a recurring character for the rest of the series but only for DevintArt because of concerns of copyright infringement if he were to appear of YouTube.