The Mazoku (and presumably, everything ELSE in the Slayers World) are ruled by the Lord of Nightmares (top circle). The two most powerful spells, the Giga Slave and Laguna Blade derive their power directly from this Lord of Nightmares. The Lord of Nightmares has no tangible form to call its own - it is more like an amorphous 'lagoon' in its natural state. However, it does tend to take on different forms from time to time as need be.
Immediately subordinate to the Lord of Nightmares are the Lords of the Four Realms (yon-kai no ma-ou). These four Lords are represented in the chart here, clockwise from the top, Chaotic Blue, Dark Star, Ruby Eye Shabranigdu, and Death Fog.
As explained in one of the early TV episodes, these "Mazoku" (literally, Race of Evil), or 'Monsters' as Software Sculptors translates to make things more confusing, are not so easily destroyed. Ruby-Eye Shabranigdu was cut into seven pieces, but was still "alive," if merely in hibernation. One of these seven pieces was sealed in the Red Priest Rezo's eyes by the King of Gods, Ceiphied. Another was resurrected by the Demon-King of the North, but was defeated by the Water Dragon Lord. The whereabouts of the other five are not known.
The primary focus of the Slayers series has been on the subordinates of Ruby-Eye Shabranigdu. Clockwise from the upper left, these five subordinate lords are Deep-Sea Dolphin, Dynast Grauscherra, Greater Beast Zelas-Metallium, Chaos Dragon Gaav, and Hellmaster Fibrizo (center).
To make the hierarchy of evil even MORE fun, each lord also typically has two primary subordinates: a general for military operations, and a priest. In the case of Zelas-Metallium there is one subordinate who fills both roles -- Xelloss is both a priest and a general.
Not much is known about Deep-Sea Dolphin, who resides in the Demon Sea, or about Dynast Grauscherra on the North Pole. Zelas-Metallium's goals are unclear - she is represented almost entirely by Xelloss, and we hardly (if ever) see her in the anime series. Her base is supposedly in a place known as Wolf Pack Island.
Chaos-Dragon (Maryuu-oh / ma-ryuu-ou) Gaav plays a more active role in the series. At least one spell, the Gaav Flare, relies solely on Gaav's power for its effectiveness. Gaav is also responsible for the genesis of Valgaav, the primary antagonist in the slayers TRY series.
"Chaos-Dragon" is how the word "ma-ryuu-ou" is to be pronounced, as indicated by the katakana over the kanji. In the anime, though, they say "ma-ryuu-ou" instead. Oh well. ;b
Hellmaster Fibrizo wished more and more control of the world, perhaps even its destruction. He gets somewhat close, in the climactic end of Slayers NEXT. He resides in the Desert of Destruction, and takes the form of a young boy.