During the Death March, the monster armies were led by beings of incredible power and influence, commonly called "Demon Lords" by those who would mention them without including a string of curses. These beings ranged widely in species and abilities, but their intelligence and strength could never be denied. Since the Death March, they've mostly occupied the ravaged lands of the East like fuedal lords, building small kingdoms for themselves.
Age: ~48,000
Species: Veirheelu
Territory: None, Doppler maintains his residence in the New Japanese Empire
Combat Abilities: Unknown. Doppler maintains an unusually powerful devil as a bodyguard, such that he rarely has to fight, but his magic abilities are clearly of tremendous ability.
Quite unknown to the Empire that houses him within their borders, Doppler is the deliberate instigator of the Death March, and a master manipulator of all that surrounds him. Powerful, influental, but above all, knowledgeable, Doppler draws up power for himself within the Empire while conducting experiments and research. It's unknown what his ultimate goals are.
Age: ~20,000
Species: Psilor
Territory: None, Demetrius currently resides within a specialized containment chamber in an undisclosed location in the U.S.A. (though several internet leaks place his cell in Phoenix, Arizona)
Combat Abilities: To most, simply beyond belief. Demetrius, as a psilor, not only commands a tremendous energy well to draw upon and manipulate at will, but is also able to attain a pure, cohesive energy state (the "true" form of the psilor). While in this energy state, Demetrius is completely impervious to physical damage, and he is able to attack quite freely while in such a form.
Demetrius Yaermon is one of the most effective generals of the Death March, and a major instigator of the great war. Since the beginning of the conflict, Yaermon led his army to complete victory after complete victory, opting to completely and immediately destroy anything he could see that was not of immediate use. He is known for two things: his devastating push during the later years of the Death March that put Europe in danger of invasion, and his stubborn insistence to never give up on the war, which eventually led to his capture. He was apprehended through a huge, complex operation that utilized the finest and most advanced magic and psionics that America had to offer, and still fought a grisly struggle that ended with many soldiers dead, and Demetrius's abilities bound.
Age: ~12,500
Species: Grend
Territory: Possesses small, but fertile and fairly developed regions in Mongolia. Also controls three gateways; two that act as convoy routes for outposts that he holds, and a third that links to the home plane of the grends.
Combat Abilities: A fiercely able warrior, most noted for his suprisingly maneuverability and near-impervious hide. His magic powers are clearly there, but his reluctance to utilize them outside of emergencies makes details of their level unreliable.
Dashtall isn't a particularly old or influential demon lord, but he is notable in that he has accomplished what several others, including other demon lords, have always failed to accomplish: the successful taming of dragons, the powerful and ancient serpents that seem to inhabit every realm to one degree or another. To that end, he has used them to expand both his territory, and his collection of dragons. His reign as demon lord ended quickly after the end of his reign as master of dragons; once Ranma destroyed the artifact he was using to control the creatures, the serpent kings flew into a rage and rebelled, smashing his lands apart as they attacked each other and feasted on Dashtall's armies. Dashtall himself was slain by Vargus, a venom dragon who he had kept as a personal attendant.
Age: ~3,600
Species: Evon
Territory: Genex maintains a small, mostly deserted spot of land in what's left of South Korea, though he's been known to set up shop and do his work anywhere he's needed.
Combat Abilities: Unknown. Though clearly a necromancer and alchemist of superior ability, these skills seem to be of a non-combative nature. In addition, Genex's mental incapacities would likely hinder effective combat decisions.
Genex is widely regarded as a fool and a madman. And unlike in the case of a certain veirheelu, Genex's detractors are absolutely right. Genex is primarily a manufacturer of zombies and golems, having devoted his superior lifespan to the study of and creation of magic-based servants. He excels in the creation of advanced undead, which require a notorious amount of effort and study. It was he that created the jakku that attacked Phoenix Mountain and Joketsuzoku, and it was he who created the hamakku, an advanced form of the dreaded zombie assassin. Despite his ability with creating servants, Genex is quite mad and foolish (it is widely surmised that he has let himself be exposed to too much methane and other chemicals during his work), and often lets himself be led around by more competent demon lords, such as Doppler Thaeramon. On that note, Genex is technically not even a demon lord (give that he's an evon, not a demon), and only commands that title because of the undead army that he's created but never uses.
Age: ???
Species: Phoenix (demigod - Saffron maintains a definite divine status, which puts into question his origins. Doppler Thaeramon theorizes that Saffron is actually the bastard offspring of Gona, an ancient of fire)
Territory: Phoenix Mountain and surrounding terroritories in the mountain ranges of China, including lands formerly belonging to the Musk
Combat Abilities: Possesses speed and strength difficult to match, as well as an inexhaustible affinity for fire magic. Can regenerate normally "fatal" wounds without trouble, and even if killed, will return to life if his regenerative abilities are not neutralized. The Phoenix Stone has amplified Saffron's natural abilities so that he maintains his adult form at all times, at greater than full power.
The master of the Phoenix tribe in China, Saffron used to be a lofty but rational ruler that tended to mind his own business. However, due to the influence of a magical artifact, Saffron grew more powerful and began to slowly grow more aggressive, seizing land for himself and helping the demon armies wipe out the nearby Musk. Saffron has since then been restless, but mostly satisfied with his territories. Unfortunately, a number of recent attacks by the invisible zombie assassins, the jakku, threw his lands into chaos, and threatened Phoenix Mountain itself, forcing Saffron to take action to protect himself and his territory. Saffron was killed by Rayden Shikodan while the Phoenix King was engaged in a battle with Ranma Saotome, right before his entire kingdom was overrun by Genex Karl's undead army (under Doppler's command).