The Demon Apocalypse was a cataclysmic event that began when Fennekin used the Whip of Binding to bind the Orb of Endless Power to the Brazier of Worlds, causing a massive portal to the abyss to open up, causing a massive army of demons to come pouring through. Lots and lots of many people died. This was possible due to the semi-knowing involvement of the then un-named Ätlän-tã Fælcons They were unaware of Fennekin's true intentions when they brought the items to him.
The Fælcons then had to find the Adherent of Erathis, then the Demonomicon, then the Hammer of Sundering, and eventually they are able to destroy both Fenniken and the Brazier of Worlds, because a noble hero, Thom the Dragonborn, sacrificed himself, perhaps unintentionally, using the aforementioned Hammer, closing the portal and ending the Demon Apocalypse. I can't believe that was all one sentence.
OOG, it lasted from Episode 79 - I Am Sorry to Episode 154 - The Battle Above the Abyss Part 4.