A Demon is a very powerful being and is not to be confused with a lesser demon who is not capitalized.
The Demon's name might look like a mathematical expression, the Demon X(A/N)TH for example, and usually resembles the place where the Demon rules. There are simpler forms of Demons' names. The simpler name of X(A/N)TH is Demon Xanth.
A Demon lives for what may be forever and can do what may just be everything. But being omnipotent and omniscient can result in boredom. For this reason, a Demon often plays games with other Demons and commits to certain trivial penalties for loosing, such as remaining in one place for thousands of years. A Demon can be any gender depending on mood such as male, female, shemale, anonymale, or just neuter.
A Demon is a major power that is, and all mortal creatures are like ants. A Demon has tremendous power and can do anything desired. No magic is beyond one's abilities.
There are three types of Demon, Major Demon, Dwarf Demon, and Minor Demon. A Demon can be demoted to Dwarf or Minor upon losing too many status points.
Xanth, named stylized as X(A\N)th, rules the Land of Xanth. Later on he has a son named Nimbus. At the beginning of the series, Xanth is at the bottom of the hierarchy and losing his status very quickly simply because no one believes in him. He/it is responsible for magic, and as a result all magic in Xanth is a result of trace leakage from his imprisoned form.
Mercury rules Mercury. Very little in known about him.
Mars, named stylized as M(A/R)S, rules Mars and is responsible for electromagnetic force.
Jupiter, named stylized as JU[P\I]TER, rules Jupiter. He is responsible for strong nuclear force which binds atoms together
Saturn, name stylized as **SA<<T/U>>RN**, is said has the highest status. This demon rules Saturn and is responsible for dimension, which is the measurement of the universe. Without dimension, the other forces and concepts can not be quantified or clarified, making it so that without it, the other demons bailiwicks are useless.
Neptune, name stylized as NE_P/T_UNE, controls the planet Neptune and is responsible for mass.
Fornax rules a domain in a distant cluster galaxy and is responsible for contra-terrene matter, more commonly known as anti-matter. She is married to Nemesis, and they adopted Santo boy from future Xanth.
Nemesis rules the brown dwarf Nemesis, just outside our system and is responsible for dark matter, which comprises roughly 80% of the universe. He is married to Fornax, and they adopted Santo from future Xanth.
Litho is the Demon who created Earth and introduced in Vale of the Vole.
Pluto, name stylized as P|L/U|TO, was once a major Demon who rules Pluto and Hades. He was demoted to dwarf Demon status because he lost a bet. In a fit of rage, he broke the cable connecting Xanth's OuterNet to the Mundane internet. He bet Eris the cable would not be fixed and lost.
Gaia, named stylized as G(A\I)a, co-rules Earth with Earth. Now the Heir Apparent of Earth.
Eris is a dwarf Demon who rules Eris. She made a bet with Pluto long ago on whose planet would be found by Earth astronomers first, hers or his. She lost and was locked away in a secret area under Castle Roogna. She took the form of Button, a ghost. She was eventually freed after making another bet with Pluto on the fixing of the outernet cable.
Gambol specializes in games. He built the playground the children from future Xanth found.
These demons appeared in Golem in the Gears. It appears any concept could have an associated Demon.
Death's name is stylized as D(E/A)th.
Depth's name is stylized as D(E/P)th.
Tenth's name is stylized as T(E/N)th.