Where does Xelloss stand in the mazoku hierchy you ask? Well, lets take a trip to the world of nightmares...
In Slayers, there are four different dimensions or worlds. The Lord of Nightmares created in each world two gods: a good one (a Holy Dragon) and a bad one (a Mazoku). The gods are equally powerful and are constantly fighting against each other to bring peace or chaos to the world they inhibit. The four Dark Lords that Lord of Nightmares created are Dark Star, Death Fog, Chaotic Blue and Ruby Eye Shabranigdu. Ruby Eye is the Dark Lord that inhabits the world that Slayers takes place in and fights against the Holy Dragon Ceified.
Each of the Dark Lords have 5 servants under them. Shabby's servants are Hellmaster, Deep Sea Dolphin, Chaos Dragon Gaav, Greater Beast Zellas Metallium and Dynast Grausherra. Each of them have a general and a priest that serve them.
Zellas Metallium, however, has combined both her priest and general into one- Xelloss. Xelloss is 2nd in command to Zellas and remains under her command on Wolf Pack island, a place not even listed on the maps. Xelloss is the strongest of the underlings, and the strongest mazoku alive with the exeption of Zellas and the 4 other mazoku chosen by Shabrindgo. If you want more info on how Xelloss came to this role, read the section entitled Past of The Priest