Four Great Satans

The Four Great Satans (四大魔王, Yondai Maō) were the original rulers of the Underworld and leaders of the Devils. They were named Lucifer, Beelzebub, Leviathan, and Asmodeus.

Summary
At some point in the past, the four original Satans led the Devils against God and the Angels in Heaven as well as Azazel and the Fallen Angels in the Great War, the latter of whom retreated prior. The Four Great Satans eventually died in the Great War after more than half of the 72 Pillars as well as a majority of the armies of Devils that serve the High-Class Devil Clans of the 72 Pillars were wipe out.

After their deaths in the Great War, a group of the descendants of the Four Great Satans intended to carry on their legacy by gathering the strong supporters of the original Four Great Satans and finish what they started, the Great War. Their intention however, were opposed by a certain group of Devils, the Anti-Satan Faction, who wanted to ensure the survival of the Devils as a species, thus disapproving of the continuation of the Great War. This dissension eventually caused a civil war to break out between the Devils which ultimately lead to the defeat of the Original Satans' descendants. Following the defeat of the descendants who were forced to retreat to the back part of the Underworld, their names became titles for the new rulers of the Devils while the Original Satans became known as the Old Satans.

The current Satans are Sirzechs Lucifer (of the Gremory Clan), Serafall Leviathan (of the Sitri Clan), Ajuka Beelzebub (of the Astaroth clan), and Falbium Asmodeus (of the Glasya-Labolas clan), four Devils who were born with immense power, equivalent or superior to the original Four and were chosen for the Satans' positions despite not carrying the blood of the original Satans.

Three of the descendants of the original Satans – Katerea Leviathan, Shalba Beelzebub, and Creuserey Asmodeus – have allied with the Khaos Brigade's Old Satan Faction, while Vali Lucifer formed his own team in the Khaos Brigade. The last known remaining descendant of the Four Great Satans, Rizevim Livan Lucifer, the son of the original Lucifer and Vali's grandfather, eventually became leader of the new Khaos Brigade, Qlippoth before being killed by Fafnir. As of Volume 20, Vali is the only survivor among the known descendants of the original Satans, with the remainders being killed in battle.

Maou Sentai Satan Rangers
Maou Sentai Satan Rangers is an informal group formed by the current Four Great Satans and Grayfia. The concept of this team is based on the Tokusatsu hero team, Super Sentai. The Four Great Satans and Grayfia first appeared in this appearance in Volume 8, serving as the examiners for Issei and Rias at the Ruins of Connection.

The rangers' role assignment are:

Trivia

 * Azazel stated the reason why more than half of the 72 Pillars went extinct is due to the ideas and ambitions of the Original Satans which is what Creuserey mention in Volume 6 to become Kings of the World by annihilating all Angels and Fallen Angels and destroying the World to create the Old Satan Faction's definition of a new World of Devils.
 * All four of the current Satans have legendary creatures in their group/peerage.
 * The current Four Great Satans are not allowed to have a harem.
 * The surname of the current Four Great Satans will not be inherited by their offspring and that surname is more like those of managerial titles as shown in Sirzechs and his son Millicas' case.