Demon King

The Demon King was the leader of the Demon King Army and the primary objective of the Hero Party's ten year quest. He waged war against the human kingdoms of the continent from his castle in Ende for thousands of years before being slain by Frieren and her comrades. Despite his importance in the world, relatively little is known about him so far. However, given the nature of demons, he must have had quite possibly the most amount of mana of any demon ever to exist.

Demon King's Castle
The Demon King's castle is located at the "northern most tip" of the continent, Ende. It is also the alleged location of Aureole, a place regarded as "heaven" or the afterlife "where souls rest". It is an insanely cold place. According to Eisen, "much has happened there", alluding to events as of yet unrevealed in the manga.

Frieren found a Darkness Dragon horn in his castle, leaving it in Himmel's possession. Himmel thought it gave off an ominous aura. He kept it in a drawer in his home for the next 50 years as a treasure, safekeeping it for Frieren.

History
One thousand years before the era of the Hero Himmel the Demon King ordered his subordinates, including a general of his army and at least several extremely powerful demon mages, to "massacre the elves". Frieren's village was destroyed and its population killed, leaving her as the sole survivor of her village. Ironically this led to Frieren meeting Flamme, who would go on to train Frieren and give her mission of killing the Demon King, which she succeeded at many centuries later.

Approximately 20 years before the Hero Party defeated him, the Demon King Army's offensive intensified, leading to a large increase in the number of mages in the overall human population. It was common to pass by mages walking through the streets of any regular town. However, by about 29 years after Himmel's passing, mages had become a rare sight, only spotted in specific magical towns such as Außerst.

Following his defeat, nearly all his direct higher ranking subordinates disappeared without a trace. However, the remnants of the Demon King Army were still a significant threat to the human kingdoms, and almost a century after his defeat still constitute large enough force to wage wars against human armies, and hold the northern part of the continent. Notable demons of high status from the Demon King army to appear so far include Qual and Aura, both killed by Frieren.