Queen Allison's invasion
In Conquest of the Underworld, Queen Allison Gryphonheart of Erathia sets out to rescue the soul of her father, Rion, which was taken from Paradise by the demon Jorm. When she sends out word that she wants brave knights to lead the charge into the Underworld, the only one who answers her call is Tarnum, the former Barbarian Tyrant who was slain by her father, but returned to life by the Ancestors to do good deeds to make up for his atrocities. Disguising his identity, he takes command of her army and invades the Underworld.
There are several levels in the Underworld, and the further down he goes, the more demons there are, and the greater the danger becomes. Queen Allison and Tarnum decides to seek the aid of Duke Deezelisk of the Bottom, a powerful demon lord who was blinded years before during a failed invasion of Erathia. They hope that if they give him the Pendant of Second Sight, a powerful artifact that can cure his blindness, he will help them find Jorm. When they give him the Pendant, he tells them where Jorm is, and gives them a large army of demons so that they can defeat his forces.
But once Jorm has fallen, the demons betray Queen Allison and her knights, and she is taken away to his torture chamber. The knight who blinded Deezelisk was Allison's father, and Deezelisk has orchestrated the whole affair to get his hands on Queen Allison, planning to torture her to get revenge on her father. Tarnum was attacked by the demons and left for dead, but returned to life due to his immortal nature. He locates Norvan and Trynn, who are survivors of a previous invasion of the Underworld, and rescues his Captain of Information Mensor from the prison that the demons placed him in. Together, they defeat Duke Deezelisk and free Allison.
The entrance to the Underworld in Antagarich is the Cerberus Gate, which allows anyone to travel to the infernal realm. It's located near Erathia, but heavily defended by demons.
The Underworld is made up of large caverns filled with resources, and demons that happily plan ambushes on anyone that enters their lands. The demons of the Underworld spend a lot of time fighting against themselves, and have no loyalty to anyone or anything - the only thing they respect is greater strength, and the only reason that Duke Deezelisk remains in power after being blinded is that none of them are brave enough to challenge his rule.