The Demonomicon of Idwilvv was a powerful book that contained spells relating to demons and portals. There was only one in the whole world. Mordenkainen gave it to Rhavar's party to protect them from demons and to use it to close the Demon Portal that fueled the Demon Apocalypse. Their party was attacked by Fennekin, prompting Steve Meloncamp to jump through a portal to "safety". He was actually sent to the bottom of the ocean floor, where he was immediately crushed under the immense pressure. His soul, however, was transferred into the Demonomicon, where it remained.
After Lord Titus Harper, Aludra Wyrmsbane, and Thom Vidalis were brought back to life by Erathis, they met Jaela, the Adherent, who told them to find the Demonomicon. Harper was given an amulet that was meant to point him in the direction of his mising sister, Adira Harper, but they learned that it was, instead, pointing to the Demonomicon in the middle of the ocean. A magician named Zird the Arcane made a submarine for them, the Belefonte, which they then used to find the Demonomicon. Thom Vidalis volunteered to go out and get it, and found it being held by the corpse of Steve Meloncamp. He took the book back to the ship, whereupon he and the others learned that Steve's soul had transferred into Thom's mind. The two now occupied the same body!
After acquiring the cursed book, Zird tells them that they could get it deciphered by Avery T'léas, in Fairmire. They go there, and Avery reads the book, learning of a way of destroying the portal letting the demons in. It is unknown what became of the Demonomicon after this. In Episode 117 - The Breakfast Club, Riekert Svenson asks for the book, but the party doesn't have it. Presumably it was left with Avery T'léas, who went missing after the White Dome was destroyed, as seen in Episode 150 - Edge of the Abyss.