The first 3 ancient demon lords:
Guy Crimson - demon race, dark emperor, ruler of the icy north. Was summoned to destroy the enemy nation of the summoner. Afterwards, he decided to destroy his summoner's nation and became true demon lord
Milim Nova - dragonoid race, tyrant of destruction. Daughter of a human and the first of the four dragons Star King Dragon Veldanava. Offspring between dragons and humans is now taboo. Veldanava became Milim's dragon assistant and was killed by some country. Milim destroyed that country afterwards and became true demon lord
Ramiris - fairy race, labyrinth fairy. Ruler of spirits, restored Milim's sanity when she was on a rampage after the death of Veldanava. Absorbed evil magic of the demon race (Guy) and powerful aura of dragon race (Milim) and became true demon lord
The second generation demond lords: heaven's gate protectors
Dagrule - giant race, wrath of the earth. Due to holy power ingrained in his body, rejected the demon lord seed at first. Awakened as a true demon lord when the previous wars started eliminating fairies and giants.
Ruminas Valentin - vampire race, queen of nightmares
Dino - fallen angel race, wandering ruler/sleeping ruler
The next generation demon lords:
Leon Cromwell - former human race, blonde demon
Karion - beast race
Clayman - undead race, marionette master
Frey - harpy race, sky queen
Rimuru - rookie slime demon lord
Other dragons:
Ice Dragon Velzado - Storm Dragon Veldora's older sister, Guy Crimson's subordinate/partner, one of the four dragons
Main story is now complete at 10 volumes (i know there are side stories tho). This chapter is around the end of volume 4. Some things are different. In the novel, Veldora and Shion went as Rimuru's subordinates. Rimuru already met Dagrule and Dino before the meeting too. Ruminas also attended the banquet and the guy in the manga who is supposedly the new demon lord who replaced her was her butler. Seems they've switched.
There are many small differences but the main idea is still aligned with the novel.
My replies got deleted for some reason when I edited the main post but I wanted to mention that there are differences in the novel and manga: veldora and shion went as Rimuru's subordinates not ranga and shion. Rimuru already met Dagrule and Dino before the banquet. Ruminas (the girl woth strong powers pretending to be a maid) attended the banquet, not the other guy and also, he was her butler. In the manga, it seems they switched. Ruminas is the maid and the guy is this "new demon lord"
There are many more small differences in the manga and the novel but the main stories are aligned: the walpurgis banquet is in action and shit is about to get down. that's all.
If still interested the web novel is available below
The chapter is now at Volume 4 Birth of a Demon Lord Arc Chapter 81
https://drive.google.com/drive/folders/1SP2StQBOpqXUf41XTv9fkjfxsUiM11og?usp=sharing
Credits to Circus Translations for translating the WN with permission from the author Fuse: http://gurotranslation.blogspot.com/p/ioduction.html?m=1
but the point of omegaverse is babies